Just to add a couple more cents:
Arches really are a natural shape for a door. Your feet are usually about as wide as your whole body, so you want the widest part of a hole at the bottom, with the width consistent some way up until the body/head narrows. It’s the same basic geometry for arched human doors as it is for chewed mouse doors. Cutting out square corners at the top of a doorway takes extra work that isn’t really needed (especially in the case of mice).
For what it’s worth, I’ve seen mice chew holes through cardboard when I used to raise them. We’d put small boxes (like Kleenex boxes) in the cage. They would either shred a box entirely and carry it off to their nest or they’d start a nest in it. In that case, they would create some holes to make getting in and out easier. These were certainly not cartoon-style arched doorways, but they were rounded holes with flat bottoms.
If you’re going to imagine bedroom furniture in there, the hole is probably the most accurate part of the picture. 