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.
But tags have the exact same issue as the current solution, which is that someone has to set up what tags the user wants to see and the only one who can set them up is the user itself. The current solution already does a better job at solving the problem than tags would and people don’t use it. If the people don’t use the current solution why would they use tags? Why not just improve the current solution so people would use it?