I agree with the "undifferentiated gossiping mass" bit.

Any specific example has a corresponding counterexample. Padma Patil, for example, gets nonzero development and, IIRC, perspective time, which could go a ways to counter "undifferentiated gossiping mass" - but a male character on about her tier of importance, like Blaise Zabini, gets to enact plot and is more distinct as a single person than she is. Even Ron, who is of negligible relevance, has a named skill that differentiates him from the background. Does Padma? As far as I can recall Padma is just sort of generically informedly bright. Hermione's intelligence, gratuitous perfect recall, and magical prowess can go a ways to counter "female characters are less competent" - but the most competent characters, even if you don't count the protagonist, are all male.