Only game I can think of where the protag is a bimbo by default (as opposed to being turned into one over the course of the game) is
Meltys Quest. The titular Melty is buxom, kind, simple and liberal although she is never refereed to as a bimbo explicitly.
There are games like
BMO TV where the central theme is trying to transform others into bimbos while avoiding (or not) the same fate.
Defenstration has an extensive bimbo route. Arguably more of a VN than an RPG.