But Daphne doesn't have any real relationship with Dolohov (or any of the other professors, as far as I can remember, anyway?) and surely Padma wouldn't expect that the scoring on exams was biased in favor of a half-blood.

I do think it's likely that she'll think she was being punished by not getting Head Girl, and that Linus was being favored unjustly, but... I suspect a lot of people might think that about Linus, unless Dolohov says convincingly that he didn't want to burden fellow Councilwizards or something, because Seamus thought it was him and Macmillan, and Padma agreed it was probably one of the two of them.