If the "Muggleborns-are-weaker" theory is true, then it makes sense.

Pretty sure this theory has been unambiguously dismissed both in canon and in MoR.

I think both have been silent on the question of whether there is any notion of inherent "power levels" at all, let alone whether it is heritable or whether it is correlated to being a "muggleborn".

EDIT: It's clear in MoR that - if Harry's hypothesis on magic heritability is true (a big if), then other non-binary factors seem unlikely to be correlated to being a "muggleborn". However, I felt that Harry very strongly anchored on that hypothesis, which was one of my reasons for being annoyed with him and eventually stopping reading (to pick it back up later on, obviously)