The hate seems wildly out of proportion, yes. (And since making the original post I've now seen the first episode of "Four to Doomesday," in which Adric does make a breathtakingly sexist remark.) He's flawed--he's got a boy's arrogance and insecurity--but he's not loathesome.

Doctor Who fandom has a tendency (which may not be unique to Who, but I find it more noticeable here than in other fandoms) to stereotype certain characters so ferociously that the stereotype overtakes canon. Like the idea of Jo as a useless screamer.