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.
Downvotes discourage reposts more than anything.
Reposting is not a problem, it’s an NSFW server, nobody will downvote repeated content. What was getting down voted were dudes and chubby women showing off.
I cannot affirm with certainty there is correlation, but the main page is far more diverse in people and content since the removal of the downvote, and constant reposting is not a thing I see (at least for now).
Reposting is a huge problem on the NSFW instance, you can see the same picture/video on multiple communities with the same title or a small variation, sometimes it doesn’t even fit the community, but those are the sellers that they were trying to protect… Along with their cringe commenting bots.