That's not because the judge is Mexican, that's because the judge is human and a huge propaganda machine was working against Trump telling everyone that he hates the judges culture.
I don't know how impartial I would be if i thought I was ruling against a hardcore racist who hates my culture.
Well, I hope you're not a Judge then, eh? ;-) (You're supposed to recuse yourself in a situation like that, you know.)
Even if he was trying to impugn human nature and not Mexicans, he's still saying that race trumps Justice (in the person of the Judge.) That's not the values I want my President to treat as normal, because it effectively promotes racialism.
Certainly his election has already emboldened racists to make physical and verbal attacks across the country. Do I care if he's really racist or not?
I don't know how impartial I would be if i thought I was ruling against a hardcore racist who hates my culture.