Even if he did hate them and target them because of it, so what? Writing every single character we ever see as being 100% completely accepting of everything would be dishonest. Maybe he just picks them at random. Maybe he just picked them due to Jim being in a mixed relationship for the sole purpose of impacting Jim even more. Who knows, no one does. We'd all be making baseless assumptions until the writers answer that question.
The funny part is, when most people saw this we went "wow he's about to kill that married couple" that should be the normal thought. Not "he's about to kill that interracial couple." We should just see them as a couple. Not a gay couple, or a mixed couple. So ironically the SJWs that bring attention to this like you're doing is ultimately counterproductive and doing more harm than good. In the sense you're just slowing down the normalization of this type of stuff. Since you're ultimately classing them and you're bringing special attention to the fact they are a different type of couple whilst also making assumptions that he dislikes them.

i feel you are overthinking this too much, as if the mad hatter needs a reason to kill anyone
The more personalities you have the less boring you are!

They are more common than WM/BF.
In Gotham? Who knows? Perhaps Tetch simply checks to see if the woman has darker skin from the man. Vale kind of has that Cambodian, Filipino, New Zealander or Native American look too. A lot of Black women do also, but that all gets into other complications how to draw "racial" lines. I guess, for some, the bottom line would be that Vale isn't White.

Wouldn't he seeking out WM/AF then? They are more common than WM/BF.
Where are the Asian characters DC WB?

This is the second interracial couple he's killed
It's because of the relationship between Gordon and Vale, a man of majority descent and a woman of minority descent. He will keep messing with people who remind him of his sister and of this adversary, none other than James Gordon.

I wonder if it'll be an interracial gay couple next.

Konrad, I haven't noticed that. I'm a conservative by the way and certainly don't have an issue with interracial couples. None of my friends who are conservative feel that's an issue. Archie Bunker is dead :). Also he was a Democrat

According to half the people on this board, the show only has interracial couples to be progressive and pander to liberals, so I think it's highly unlikely Hatter is killing them for that reason

LOL I noticed that as well.