The tag implication #57495 diaper_only -> mostly_nude has been rejected.

Reason: Following the same implications as underwear_only

EDIT: The tag implication diaper_only -> mostly_nude (forum #394693) has been rejected by @dragomess99.

mostly nude: "A clothed character with a completely unclothed torso and groin."

underwear only does not imply that and this shouldn't either.

underwear_only does not imply mostly_nude.
mostly_nude is for when a character is wearing some form of clothing but their torso/chest and pelvis/groin are mostly or entirely uncovered. if a character is wearing some form of underwear (or indeed, a diaper) they're going to have one of these two places covered.