The thing is, I don't think you're getting any kind of feel for the character at all when you try her on a team that is so fucked by stacked Ana's that there is nothing any of them are going to accomplish except get wrecked.
To play Ana correctly, you need to
1. Find and maintain some kind of cover using the map. This is not something you are going to do while getting steamrolled because the enemy team is going to shoot you out the moment they're aware of your position.
2. Learn how to fire and distinguish targets. This is not something you are going to do with 2 other Ana's who are in the back lines with you. Even if you are doing everything completely right and you're healing like crazy, a 6 v 3 match is not one that's likely going to last long. What do you learn when the result of skilled gameplay is identical to bad gameplay?
3. Use your ult smartly. Again, steamrolling basically nullifies this. No matter who you are, even a nanoboosted Reinhardt is going to fall when everyone concentrates fire on him.
I really feel that the only way to really learn how to play a character is to play them in a normal gameplay environment, because that's where they're going to be used anyway, once the ana fever dies down. Now, you might have more of a point if this was literally any other character class. But this is a steam game, and especially as a support sniper with limited mobility, you're kit basically relies on the rest of the team to be effective.
Look, trust me, I get it, I'm sensitive to the idea that people want to just try out their favorite character. I get that. But stacking in all it's forms is a cancer. Stacking your support sniper with limited offensive and mobile capabilities is like a 9/11 of cancer with a side of the holocaust. Not only is it essentially throwing away the game, something no one likes whether it's QP or ranked, but it's also not in any way conducive to learning or enjoying the character. I would much rather have a person try her out in Ranked than 3 people in QP, because even if their entirely incompetent with her, then atleast the team will be down only 1 player instead of 3.
There is a difference between learning how to use a character's kit and learning how to use a character effectively as part of a team.