Many (most?) instances do block porn. Aside from liability concerns, porn tends to be high bandwidth, meaning high costs to operate. As such, there are a handful of VERY NSFW instances that aren’t well-federated.
Of course, it depends on how you define porn. I see a ton of suggestive (but not outright pornographic) anime when I browse All/New, but very little of actual naked humans.
Unfortunately, that seems to be about local communities, which aren’t really important. In fact, due to federation, that’s the least important aspect of an instance.
By default, Lemmy is wide open to anything and everything. You need to see what they have turned off/blocked/removed, to see if it’s what you want.