Personally, I think "fat" was just a description. It could also mean to describe them as spoiled, (think of Harry Potter, when people accused JKR of being anti-overweight, because Dudley was fat, because his parents SPOILED him) but also simply as a despription. But spoiled kids are often both fat and mean, because they get more food and more attention and rights, etc. At least in that time. A skinny child has less money, is less spoiled and therefore friendlier. That is very simple, of course, but I interprete it that way. Apart from that, Lewis himself was quite a large man in his later life. Don't forget that. And his love Joy Davidman used to be quite large, too, until she lost some weight, when her sister suggested it.
The same goes for him talking about non-smokers, vegetarians and teetotallers. I don't think it was menat against these groups or that he wanted to show them in a bad light - he was just talking about these people (the Scrubs) being very, very "modern" (in their view). The vegetarian is a wonderful example Eustace and his parents did NOT like animals. But they were vegetarians - to be modern. Not because they had a point (except health, of course). He did not talk bad about these people (vegetarians, teetotallers, non-smokers) in general, but people who desperatley try to be as modern as they can, without even having a point for doing so. Even though there are points for being that. But just wanting to be modern is obviouly not a valid reason, from CSL's (and mine) point of view.
As for Susan... the lipstick and tights only meant her interest in material things was larger than her interest in anything else. He could have also used money or brushes (for hair) or dresses. Of course, his examples were quite clumsy, because people today often assiociate it with sexuality or anything like that (which is pointless, because if people who don't have sex could go to Narnia, only children and celibate people could go there...) but it's not the basic meaning. It's simply about materialism. Not about being pretty (or making yourself "prettier") or socialising or anything like that - as long as it#s not the only thing of importance of one's life.
And please, excuse my bad English, I'm a little tired and not a native speaker...^^"