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.
As I understand it, the reasoning is that someone posts a picture of their gaping arsehole and it gets downvoted because of the horrid hairy mole on the left cheek, it might dissuade them from posting pictures of their naked breasts — which might be more in line with the community’s aesthetic sensibilities — in the future.
Which, frankly, seems kind of reasonable.
Lemmy does need more user engagement. Even if you discount the 99% of posts by repost bots, reddit’s maggoty corpse still has one or two orders of magnitude more user engagement, which means more user engagement and more tits and gaping arseholes, even if some of them have moles. Anything that makes people more comfortable posting original smut (and other kinds of content, I suppose) here is welcome.
That said, I think this only should apply to posts, not comments, and only in specific communities. Downvoting gaping orange arseholes in political news communities, for instance, should not only be allowed but even encouraged.
(Personally I still downvote everything anyway; it might not affect the vote count, but it makes me feel better.)