Downvotes are important to ensure quality content. It allows the community address statements made by a user based on objectively incorrect (mis)information. This feature is an important reason why many reddit users aren't on Mastodon. Also, democracy is important.
Recommended Instances shouldn't wholesale block content just because it's NSFW. As you say, policy on what NSFW content is allowed is distinct from the instance enabling NSFW content.
People being able to create and moderate their own communities is positive
If an instance (eg Hexbear) wants to deviate from this, that's fine. That's what the Fediverse is all about :) But we shouldn't recommend those instances to new users as it will cause new user attrition.