Ok, so first off, from my personal experience Mei's wall isn't all that useful. It can sometimes catch someone out but if you were playing a real damage class you could just kill them rather then isolate them. Just cut out the middle man. The best use for it I've found is to block defender's line of sight when pushing in. But even then it's not all that much better then Reinhard's barrier which can last until it takes 2k damage and can also be shot through by his allies and moved around. In theory it sounds insane but in practice it seems very situational.
Her ice block is probably the most useful part of her kit but even then it's not as useful as you're making it out to be. Comparing it to Zarya is laughable since both her self-projected and ally-projected barrier don't prevent them from dealing damage. It's a very good ability but at the end of the day when the ice thaws you're still playing Mei, one of the worst characters in the game. (More on why later)
You are underestimating how much of a disruption her barrier and wall are. Think about how many abilities in this game are designed to move an enemy: Pharah's rockets, Winston's ultimate punches, Roadhog's hook and ultimate, Lucio's sound blast, etc. It completely drives momentum to a halt. As good as Zarya's barrier is, it doesn't hold a candle to Mei's ice barrier.
Her long-range damage is a joke and pales in comparison to anyone that has a weapon that can shoot far. Even Lucio's poke is more threatening. 4 headshots with Lucio's sound blaster deals 128 damage. Mei's max range icicle headshot deals 44. If it was before they gave the gun horrific damage dropoff I would potentially agree with you but now her ranged poke is a joke. There's nothing that she can dish out at range that can't be quickly healed up by a Mercy. (One of her biggest counters, for reasons I will explain) Not to mention it's very inconsistent given that it's a projectile and not hitscan, making it a lot harder to hit with at longer ranges.
Mei has the potential to stack up damage from a reasonable range better than others. It is an extremely fast projectile, father than most others in this game, almost to the point of being hitscan. I don't know why you are purely comparing damage numbers and not projectile speed. It is nearly impossible to hit 4 headshots as Lucio if the enemy aren't, as you say, headless chickens.
Now on to why she's bad. What's supposed to be her biggest strength? 1v1ing? Well here's the problem with that: Assuming the enemy team isn't just 6 players running around like chickens with their heads cut off, the only characters you'll be usually 1v1ing are roamers like Tracer, Genji, Winston, and Reaper. And pretty much every common roamer in the game either beats her in 1v1s (McCree and Reaper both demolish her provided they play well) or can easily escape her if they see her. (Winston, Tracer, to an extent Genji) She has little kill pressure roaming because there's plenty of people that can beat her and the ones that can't can just run away. And she lacks the explosive power of someone like McCree or Reaper or the mobility and burst of someone like Tracer and Genji. So even if she does get a flank off it's not as powerful as many other characters.
Tracer can dash out of her freeze beam but maintains her freeze timer. Not to mention, before Tracer can even get into range to do serviceable damage, an icicle will OHKO headshot her. Genji cannot reflect the ice beam. McCree will only win if he gets the drop on her OR he's close enough to flashbang, otherwise he's frozen and dead. Winston does outclass her in health but not damage. If you jump in as Winston and miss the damage on Mei, you are dead. Sure you can snipe Mei with Hanzo or Widowmaker, but she has easier headshots on you since she is moving and you're still.
Ok so if she doesn't do that well in 1v1s how is her teamfighting? Not good either. The reality is that her whole kit is entirely focused on picking off one character at a time. Her fast burst combo of freezing into headshot shard deals around 200 damage. Which is good for people that only have 200 hp but anyone that has more then that things become tougher. From there she either has to try to land more ice shards on unfrozen targets to finish them off or go for a refreeze. Neither of those are really desirable. So people that are being healed by a Mercy or are just naturally tanky and have lots of Hp take a fairly long time to kill for her. In an isolated 1v1 this might not matter as much but in a teamfight her low dps can make it fairly difficult to actually kill someone quickly, giving their teammates ample time to force Mei to back off or die. And Zarya is even worse for her since Zarya can just shield the target she's going in on, removing the freeze and making them immune to Mei for 2 seconds, forcing her to start over after the freeze ends. And Mercy's healing stream makes it extremely difficult for Mei to finish the person she's healing off. Try to kill a Reinhardt being healed by Mercy as Mei. It's not possible. And unlike most other pick-based characters, she can't just freely switch between targets thanks to how long her freeze takes to build up, meaning that teams that can protect their teammates from her can easily shut her down.
Mei can land several ice shards into a frozen character before they can move again, her ultimate allowing more so. In fact, her ultimate covers such a large range that if you're anywhere but on the perimeter, you're screwed.
I don't understand why you think Zarya's barriers are better. Yes, they allow targets to attack and move, but Mei's barrier negates EVERYTHING. Also, why are you using an example of Mercy + Reinhardt? No one, save for maybe Bastion, can do enough damage normally to counter that.
True, that she cannot do a lot of AoE damage (there are only a few AoE regular attacks), but her ultimate gives her enough time to finish off several opponents.
The nail in the coffin though is that she lacks any real mobility. Sure the ice wall can help her reach some unexpected locations but her lack of movement makes it very difficult for her to close the gap on characters like Mercy or Lucio, who she would really want to kill. Even if she does close the gap, Mercy can Guardian Angel to a farther away ally and Lucio can just alt fire into speed aura away. The things she 's supposed to be good at other people do better and she has far too many gaps in her kit for her to be effective in a coordinated setting.
She is listed as a defensive character, which she excels at. She can stop the entire opposition for several seconds at a time, close passages (unless the ENTIRE team is there to break the wall), render most ultimates and special abilities useless with her ice barrier, and ultimately, kill any forward momentum the team has. It isn't like Bastion or Torbjorn where there are explicit hard counters. She has no counters. McCree is not a counter if the rest of her team spots you before you get to her in the back. Snipers don't work if she keeps walling herself off.