Makes sense to me. Slytherin has a rep for being the House of Evil Overlords (when really, it's just defined by ambition, not by being "evil") and Catra wants to see herself that way. But she's more of a Hufflepuff because 1)she doesn't actually want power and 2)she does have the defining trait of a Hufflepuff: she's an extremely hard worker. Even when chasing after something that, deep down, she doesn't really want.

Adora is ambitious, which is apparent in how she fell right into the Legendary Hero role. There are times when she's doubted herself (namely due to a need to be in absolute control of an outcome), but she can be downright cocky about the whole She-Ra thing and arrogance is very closely related to ambition. But since Gryffindor has a rep for spawning heroes and Slytherin is the "bad" house, of course she would fight tooth and nail against being defined as the latter.