I think that’s a reaction more to media forcing the notion of diets down people’s throats and thus causing a normalisation of the idea that (a) you will have eaten more and worse food over Christmas (not convinced this is particularly the case nowadays) and (b) you should feel bad about this.
Obviously this overspills a bit into getting narky at friends going on about their gym use, I guess and that’s a shame. I don’t think keeping fit and healthy is necessarily the butt of anyone’s joke, although any person for whom a single interest is wildly consuming (equally, this isn’t aimed at you - I mean your total gym fiends) will get a certain amount of piss-taking from that, be they bikewankers, Goths, gamers or foodies, etc.