Sure you could have a small community of mestizos present in Spanish ports (Lusitania had something similiar happen in his Portu-wank timeline) that eventually melds with the Euro population.
Why Spain would do that? Hardly Spain want take risk about more ethnic problems on its soil.
Well, Montezuma's descendants are part of the Spanish nobility
Yes, its not that common knowledge, though. Would be interesting, if they had been more prominent and influetual in Spanish history. Maybe some more Aztec and Aayan nobility converts in a rush before the Empire falls and become direct subsidates of the Spanish King.
What if from the 16th century on converted Natives and so called Mestizos on a larger scale move to the Iberian Penisula and ad to the Spanish population ?
This was OTL to an extent. The Inca nobility converted to Christianity to save their hides and maintain some degree of influence in the new order. The Inca-descended nobility was wiped out/disfranchised during Tupac Amaru's revolt.