I wouldn't say thunder thighs in itself is an insulting, but keep in mind that slender thighs are part of the ideal in conventional beauty, it's probably not a good idea to go around pointing out women's thighs being thunderous. Sure some people like bigger thighs, but back in Ancient Rome, a big nose and baldness were considered attractive and perhaps some women still do find those things attractive, but I don't think many men would take too kindly to people talking about their big nose or hairline.