Would a non-slob to slob transformation story really need to have intelligence loss? And, on a somewhat related note, what sort of occupation or hobbies could a slob character have?

It wouldn’t–smart people can be slobs just as much as dumb people. However, the entire “slob” TF is for most readers (not all of them) about a certain…loss of discipline and caring. A loss of intellect slots in as a great trigger for that broader fetish, because losing one’s intellect is seen as a sign of becoming a baser kind of person. However, it is hardly necessary to include it, and a desire for a slob TF can be articulated without relying on that loss of discipline. 

As for occupations and hobbies, it would depend a lot of what kind of slob you’re creating. A smart slob could be a programmer–overweight and obese, living in a basement, snacking of junk all day and growing huge while he writes code. Hobbies for this character might be video games, or other activities that can be enjoyed alone, or with others at a distance. A less intelligent slob can be employed in any blue collar position, though usually the dirtier the job the better. Construction workers, garbagemen, and janitors work for city settings, while mechanics, farmers, and truck drivers are all possibilities for more rural locales. These characters would probably enjoy anything that lets them get dirty on purpose–sports, mudding and off roading, and camping or hiking if they are a fitter slob (just as intelligence loss isn’t required for slob TF’s, neither is weight gain). Alternatively, you can have the character be unemployed, living on disability or welfare, or living with a family member or friend as a mooch. Just a few ideas–it really can go in a lot of different directions depending on your story’s setting, MacGuffin, and the sort of slob you want to create.