I agree that there are also other groups of people who are differentially negatively characterized; I restricted myself to discussions of women because the original question was about sexism.

I cautiously suggest you could say the whole premise of lw characterizes most people negatively,

I would cautiously agree. There's a reason I used the word "differentially."

Should negative characterizations of people be avoided in general, irrespective of how accurately we think they describe the average of the groups in question?

Personally, I'm very cautions about characterizing groups by their averages, as I find I'm not very good about avoiding the temptation to then characterize individuals in that group by the group's average, which is particularly problematic since I can assign each individual to a vast number of groups and then end up characterizing that individual differently based on the group I select, even though I haven't actually gathered any new evidence. I find it's a failure mode my mind is prone to, so I watch out for it.

If your mind isn't as prone to that failure mode as mine, your mileage will of course vary.