“Mouse gun” generally has a bit of a pejorative connotation today, like “this gun is barely adequate for mouse, certainly not anything larger such as rabbit or two legged predators.” In that sense I believe it’s always been traditionally restricted to .32acp, .25acp, .22lr, .32naa, etc.
If we use the term in a more generic sense to refer to any small or highly concealable gun, ie “small as a mouse,” then we should probably include the Kahr 9mms, as well, certainly, as J frames, LCRs, LCP and KelTec .380s, etc.