Are fat jokes (ever) okay? I don't know the answer to that question, but thought about it a lot watching (finally! yay Netflix streaming!) the newest Wallace and Gromit film, A Matter of Loaf and Death.
Wallace's love interests--Wendolene, Lady Tottington--are always rather grotesque. Piella Bakewell is not a departure in that sense, but she is new in that she combines both love interest and villain; usually it's one or the other, or both, but in separate forms, threatening the primary Wallace & Gromit relationship. Piella, the former pin-up model for 
the Bake-O-Lite company, is a serial killer, seeking revenge on the bakers who ruined her career by making her fat, and needs Wallace, now a baker, to complete her "baker's dozen" of victims.The fat schtick in Loaf and Death is a symptom--or maybe cause--of how this movie isn't as clever as some of the others. Piella is really only about her homicidal monomania and her size (with a bit about her horrid middle-class taste thrown in).
That's it: set-ups, character, jokes, plot-points, whatever, are about just an evil fat woman. She meets her justice eaten by an alligator when the balloon she's in can't support her weight ("I'm as light as a feather!"). The last line of the movie is Wallace saying, "Always room for a small one!" as he invites Fluffles, Piella's dog, into the cab of the bread-delivery truck with him and Gromit.
Wallace is always oblivious to the threats in love or hate, but Piella seems without redeeming qualities at all, although I guess we are to pity her at the end for her fury at what baked goods did to her. I didn't find her or the movie that fun or interesting as a result. I could have done with much more focus on the details of W&G's baking business, including more fun Nick Park machines. Also: I would happily watch an entire film about Fluffles! With her adorable trembling knock knees comme Shaun the Sheep. Too cute. Maybe it would be too saccharine, though.
I will say this, having seen Despicable Me recently as well: you can tell animators enjoy depicting large bodies, blowing up contrast between both fat characters and thin, and the extremes within one body (such as the lady tourist, below). You can tell they have fun with it. It seems to me that you see a lot of fat in animated films these days.
It hugs a tricky line, though, the balance between parody and style, especially when filmmakers are set free-er with animation to make the world they want to see. As is usual with a fat character, the depiction of Piella is cool as well as kind of awful. You can do a lot with a fat lady made out of clay.
I'm really looking forward to seeing The Illusionist, the new film from the director who made The Triplets of Belleville, which contains the most dizzying gestures using size I've ever seen in a film--plus the colors in the filmmaker's graphic, strongly 2-D world are so beautiful they make my body temperature go up.
