Do I remember wrong or didn't Kaoru only adopt her present holy shit I'm fabulous persona after getting (not unreasonably) dumped hard by Shizuka something like two-three years back, though? She certainly hinted as much to Hibiki way back in ch 46 and 61 makes it clear she took that one very badly...
Well, not that she's exactly unglamorous in the flashbacks either but it apparently went into high gear after that episode.
Honoka would probably be quite the looker herself if she didn't affect the "disheveled fujoshi" habitus instead; she certainly seems to "clean up nicely" as it were. Not hard to theorise that's a more or less specific, if not necessarily conscious, reaction to growing up in the shadow of her older sister (and perhaps their statuesque mother too) - explicitly avoiding direct "competition" or comparison by going virtually the polar opposite way instead.