I have no interest in guns, and only a minor interest in sports and cars; but if you set up an instance devoted to these, and got your friends to use it (just to talk to each other), I reckon it would only be a few months before you'd seeded a community. (There is plenty of comedy on the Fediverse, so I think that's a bad example.)

The design of the Fediverse is receptive to niche communities. If other communities are hostile, you can just pretend they don't exist, and the things they post won't appear on your timelines. "The people on it" is not as much of a thing as you might be used to from social media like Instagram, Reddit, Tumblr, or HN. (As everyone's so very fond or saying, ActivityPub is like email.)

If your niche is a popular niche (which sports most certainly are), then it should get quite big, quite quickly, provided the people who'd participate in it are (or can be) present.

I disagree because you dont just want "twitter for MMA" you want the whole network. So you want to be a good citizen and have instances federate with you. But most wont because of the nature of the content. So users would prefer platforms where they can follow all content from a single account.