can she became muscular in the gym ?
for as long as i know no but if you want to try feel free to but just know that it might be for nothing though it would be an interesting new ending.
But if you're interested in seeing a muscolar aubrey you can do so by selecting the appropiate experiment at Tiff's lab (once at least weight stage 7).
(Note: even though you unlock the Lab at weight stage 7 as you get to higher weight stages you will unlock new experiments so don't worry if you don't find something you are looking for immediately).