@rachel it isn't, in principle. But it is in form.
On mastodon you might have servers dedicated to that type of content, the user decides what to join and followers can block the server.
Every server has its own rules, it's the user that decides in witch one to sign up/block. On top, the server admins can block certain servers by default.
Yes, you can have an ocasional pop up of some NSFW, but if it's against the rules, will be banned.