Literally, she made the cup hot because she either misunderstood the assignment or chose to do it 'wrong' on purpose.

Narratively, I think it's intended to show us Noa's character, maybe that she takes the wrong approach and doubles down when questioned, even by the instructor. Maybe that becomes her source of strength?

I'm a little worried that her title as "the muscle" when she grows up means she brute forces magic and has acquired an absurd amount of durability through layers too complex for kat to deal with quickly or something. She came around the corner ominously, alone, opposed by a character that just got antimagic lasergrids, a fire elemental with ties to multiple demigods and death, and a teleporting dragonslayer. I feel this foreshadows that something real stupid is about to happen.