By default, Lemmy allows downvotes globally. However, when a server disables downvoting, it is similar to using a feature that is usually reserved for enterprises and very small, non-federated communities.
If a user prefer to not see downvotes, they can disable it by his favourite client settings, but the rest of the community should not miss this functionality for the pleasure of few users.
I usually browse top 12 hours or scaled and I haven’t seen an actual amateur post in a long time, so I think you must be right.
I think the sellers would get upset of not being able to mix their spam outside their comms, but I agree.
Altough I don’t have hopes for a tag system for a while, being careful of the tags not overlapping accross instances sounds like a pain.
At this point, I think everyone has blocked whatever they don’t like (based on sexuality), so if that hasn’t worked enough to stop the massive downvotes I doubt a tag system would work for that (since they don’t only come from one instance and sometimes they create instances for that purpose).