I don't think it even matters that he's Dutch. It's that he's not Wout, who Belgian fans seem to have an almost spiritual connection with, and keeps beating him handily.
It 100% is because he's Dutch and little to nothing to do with him not being Wout. It doesn't matter where he was born, he has a French mother and a Dutch Father. His cycling heritage has nothing to do with Belgium. This is why they hate his dominance so much over there.
Like
@theyoungest said, if it were Nys Jr. there would be no issues. It's clear as day IMO.
The fact that makes the anger worse each year is that, objectively, there shouldn't even be talk anymore about the big 2, as their 1v1 duels and titles are so far in favor of Mathieu that it's just disingenuous at this point IMO. Only sparsely winning on ridiculously mud-heavy courses doesn't equate to being on the same level. Mathieu sits alone at the top. Tell that to those w*nkers trying to sabotage him.