Do you think Mai has low self-esteem? She came across to me as someone who has a healthy amount of self-esteem. I'm surprised to see this take in the fandom a lot.
Yes and no.
I think she considers herself to have value, and no one can make her believe otherwise. She's never going to wonder "Am I good enough?" unless she's counting enemies and trying to figure out if she can take a hundred dudes or if she should wait until about 20 of them knock off for lunch.
However, for a long time, she did not think she had the power to affect the horrible world around her in any way. She was devaluing her own impact on life, thinking the best she could do was carve a little space where she could hide a few happinesses in the shadows as long as she contorted herself into the right shape against the glare of the sun. She eventually discovers that not only can she take positive action and express herself, she can actually effect change and shape the world to her will.
So I would say that Mai's self-esteem trends up across her timeline, but it already starts with a solid and very hard core.