80 is "soft cap" for character ascensions. The gains from 80-90 are small in comparison to other levels and require a TON of resources and farming.
HOWEVER
There is a hidden mechanic that takes your character's level and compares it to the enemy level, and will adjust damage accordingly depending if you're above/below the enemy you're fighting. This starts to matter more at max world level and for combat events or abyss. You might have also noticed a big discrepancy in enemy strength and especially HP between world levels 4 and 5 (I think that one was the big jump). They might have eased this a bit for new players, but everyone rushing content at launch kinda got surprised by the jump at first, and a lot were caught with their pants down and a bunch of weak characters (myself included).
Particularly elemental reactions have a huge discrepancy for damage if you're vastly under the enemy's level vs. the same or higher. If you're doing a Swirl comp for instance, where the majority of the damage comes from reactions, it's definitely worth it to max out your character level asap. If you're doing mono-Geo or Freeze or Physical or whatever, it barely matters. If you find yourself sticking to a "main" team more often than swapping to different teams and characters, there is no downside to maxing them out. Once you have 2 full strong teams, I would argue maxing them out is more important than having 20 level 70-80 characters, but it depends on what your goals in the game are.
Weapons and talents on the other hand get pretty big advancements from higher levels, so if you're trying to ration out your resource usage, I would give them priority.
I can't find the graph anymore, but it showed general power % and what they came from, and artifacts were the biggest slice of the "character strength" pie at like 21% if I recall. The other parts like talents/level/weapons/etc were smaller individually.
Then again, this is just my opinion, but I'm like an anti-Fuz with character building. I max out a single character at a time and then move on to another one, with the exception of artifacts. I am real lazy as far as farming them because of the awful RNG at play. You can run an entire week's worth of resin in artifact domains and not get an upgrade, it's disheartening.