World Court opinions should have zero standing in the United States (or any allegedly sovereign country for that matter)
The problem is that the US wants to ensure its citizens have consular access when it is abroad, so from that perspective, the World Court is needed. I think if this were not a big election year, Congress may have intervened.
A big problem in a lot of these cases is Mexico always demands its nationals who commit crimes not face the death penalty. It doesn’t look at whether the accused got a fair trial. And, in this case, the gangbanger was only “nominally” Mexican - a Mexican citizen raised in the U.S. Mexico should not have taken this to the World Court. Lots of Americans and Canadians languish for years in Mexican prisons if they fail to buy their way out.