@ZayB: It might be so--very likely, in fact: it'd be a fine way to illustrate the hotheaded girl's hair-trigger temper. However, I think that there's another pair of reasons why it's done: first, in an attempt to appear fresh (I don't get how at this point, when it's been done for years by now), they create in such girls and their characterization a rejection of traditional ideas of beauty--the girl, along with the MC's inexplicable interest in her and/or everyone else's high evaluation of her looks, is actually an instance of the author asserting, "Hey--girls like this are beautiful too~". The reader is railroaded into accepting this if any sort of immersion is to take place.
The second is a form of virtue signaling. It extends beyond looks and includes personality--you can see this in at least two of the Shanaclones--Louise and Taiga of Toradora. Both of them have abhorrent personalities (Louise goes so far as to
savagely whip Saito--imagine seeing those wounds given by a guy to a girl), and neither of them receive comeuppance for what they do--only the MC's acceptance of all her flaws (in addition to their very few redeeming character traits) over every other (patently superior, especially in Louise's case) girl that shows up. Perhaps it's an extreme version of the assertion described above, in which the author says that the girl can be so repulsive as to abuse her lovers, but is still supremely lovely.
Anyway, I watched both to the end, but I didn't come away from either one with positive thoughts. Shana, I didn't like because the twist ending was poorly executed; Zero...just seemed like a waste of time, with Saito pledging so much loyalty to Louise the way he did, at the end. By that time, almost every girl around him proved themselves a better choice, with Louise constantly raging the way she did from episode one--supposed to be the modern tsundere appeal, I guess. And as you said, people liked it then, and they still like it now--even over instances of properly beautiful, genuinely sweet girls. Perhaps it's masochism.