Yes if you repeat cousin-couplings consistently, eg as a wealth protection strategy because you’re a European “noble”, then yeah you end up with that fucked up shit that was the Spanish monarchy and Prince Charles.

There was a Spanish king who was in fact more inbred, in a genetic / statistical sense, than had he been the product of a sibling-pairing.

Slightly inbred cousins having children with even less genetic diversity who marry their own cousins etc and the genetic pool gets about as deep the lyrics of the spice girls - when repeated as a norm for generations.