I'm aware of a couple FOSS dating apps but they're not federated. None are popular either. The problem is that dating apps really need a good influx of new users and a strong population for all of the positive network effects to keep going as people stop using the app. The fediverse is still pretty niche [citation needed] so building an app that does well isn't going to be helped by putting "it's federated" front and centre. Personally, I think it makes sense to focus on making a good dating app (like one with a good questionnaire akin to OkCupid) first and then adding federation later once the minimum features are functional. Unfortunately that makes federation hard, because it'd be a centralized platform with some inertia at that point.

I think #Fedidate and similar Mastodon tags are probably the better solution at this point. Or the significantly better option of meeting people irl.

https://parameeter.org/