Just for the two or three people who care. The reason being I found out I'm trans, and I get a slight twinge of dysphoria whenever I log in. New name will be "CuteChaosSlut". Yes, a highly dignified name I know.

And here we are!
Now, how do you set an avatar on mobile?

Weeeeeeeell depends

Do you want to set a picture as an avatar no matter what it looks like?
Or do you want to properly move and scale the frame on the picture so it looks like you want it?

If it's the former, click on use as avatar.
If it's the latter, try to move the frame 20 times, play with the zoom factor 10 times, somewhat manage to make the frame window super small, try switching to the desktop version in your browser, realize you have to click use as avatar again, then try to move the frame 20 times again. Then give up in frustration and just set the avatar on a pc once you have access to one.

A fellow trans person!
And yeah, unfortunately setting a (good) profile pic on mobile is pretty difficult. I hope you have access to a computer.