Defederation is a horrible solution to that problem imo. It would mean no other user on this instance could see any of their content, just because you don't like some of it. Worse, it cuts off all the users on that instance from all content on this instance. You'll just create fragmentation and make things worse for everyone imo.
This problem only exists for you because you're browsing All. The obvious solution is to just subscribe to the communities you're interested in. There are plenty of good ways to discover communities and stay up to date on new ones without using All. All really means ALL content, that means there's going to be stuff you don't like, and that's definitely not a good reason for defederation.
Long term I'm sure that a) as Lemmy grows, All will fill up with more mainstream stuff and you won't see so much weird niche content, and b) there'll be more advanced filtering/blocking tools so that you can do All but choose which instances to include/exclude