I wouldn't say fat people/obese people would be exactly frowned upon either, but they do stand out easier though, as majority of the Japanese are very lean/well-proportioned. What I have noticed though is, that Japanese do comment fairly bluntly about someone getting more fat. My husband has gained a rather cute belly (he used to be super skinny), so I've been next to him when people just point it out - "you've become fat!" "Oh you've grown yourself a tummy!". I reckon it's kind of teasing, in good will, rather than anyone being very serious about it. Commenting on one's appearance is more straight-forward in Japan anyways,than for example in Europe. I've also received sudden comments on the tone of my skin, sunburns, pimples, hair color etc. even from almost total strangers. It's just a cultural thing, and not meant to be offensive at all.