Now I haven't seen the latest episodes of 30 Rock, and not to defend use of old, worn out jokes, but the "Liz Lemon eats too many donuts" gag is getting attacked for some of the wrong reasons. Yes, Tina Fey is pretty skinny, so the idea that she's actually a disgusting glutton is unbelievable and even annoying. But I always interpreted this joke as related, in part, to the jokes in the "Cleveland" episode, where Fey is repeatedly praised by flyover country Americans for her model-level looks and figure.
Insofar as the message of New York City is "Be thin, be rich,"* we are in part laughing at Fey for her Homeresque consumptive habits, but also at the (absurd) idea that in her environment, Fey IS fat. Her level of indulgence, perhaps unremarkable in other locales and not inconsistent with her relatively slim body, is striking in contrast with the ultra-disciplined eating habits and lean lines of other New York women in the TV/film industry, which is the specific setting for the show. Even the more petite of us can be made to feel piggish in certain circumstances.
* I just read this quote somewhere and the original Paul Graham link is all I found. Anyone?