It's a bit insulting, as a child prodigy who has basically thrown their life away to study and train to reach the power level they have, to have to train someone who not just reeks of mother's milk but flips the process on it's head and needs to become weaker.
On top of that, there's a very good chance the brat could hurt their hero in the near future.
All in all, I think her prickly demeanor is not unusual and she'll join the flufffest eventually.