himboification in this case, though himbo is a pretty "recent" term.
I dunno about in art becase it's harder to portray mental changes via drawings, but muscle growth + dumbing down stories have been staple gay fetish for a very long time.
You can find tons of these in CYOC or Gay Spiral, or tumblr even, the tags are usually #muscle growth and #dumber.
It just appeals to the more primal/sexual aspect of masculinity, that is, guys that are "dumb" are also more likely to be cocky, muscular, think with their dicks, be sexually active etc.
Pretty much the whole dumb jock fetish.