I wonder which side more of the readers self-inserted as. It seems like it would be about equal for those that want to abuse the cute Mahiro and those that want to be the abused cute Mahiro.
This is the same dungeon that is magically always safe temperatures. I'm sure he'll be fine.
In reality, I presume the magma would offgas faster than it would cool, so it might be doable if he wasn't underground.