If Richter is obscure, than Simon is just as obscure then he is. Although in recent years, that's mostly due to Konami in general. But that aside, aside from Judgement and Mirror of Fate, Simon makes next to no major Castlevania appearances since Castlevania Chronicles and the last Castlevania appearance he's made was the Castlevania Collection that had Castlevania 1-4 and Bloodlines IIRC. And even his crossover appearances have dwindled, with the last non-Smash Bros one being Super Bomberman R/R Online last time I checked.
Whereas besides being in arguably the most popular Castlevania game Symphony of the Night, Richter is set to be the next protagonist for the Castlevania Anime, which has gotten a notable following. And Castlevania Requiem, which had Rondo of Blood and Symphony of the Night had also released not too long ago as well. And he also joined Simon in Super Bomberman R Online.
Like, if you're looking at a casual perspective, neither would be relatively known names. And if you look at the major fanbases, more than likely if they know about one, they would know the other. Castlevania and Konami fans would know both easily. And old Nintendo and Smash fans would know both Simon and Richter since the Belmont Clan name alone practically comes with a timeline and history of heroes, whether it'd be Simon, Trevor, Richter, Julius, Leon, etc. Basically, Simon and Richter would make for a good case of "You either know both of them or neither of them." And that's not even counting that Castlevania in general was a popular pick in the Smash Ballot.