Looks like it... At first I thought that was part of the side ribbon but I looked at all the others and it's there pretty consistently. There's a dead-on shot two pictures down (where Marisa attacks the cat), so yeah, looks like we have us a mutant...
Actually, I just looked at the rest of Kou's work and it looks like all of their Reimu's have ears... Can't say anything about the other varieties, since they all have hair that would cover it.