Obviously, all other things being equal it's better to be placed with a white (or, any) foster parent than none at all, but I don't see why it's necessarily flawed to view being Hispanic as a potential advantage for a foster parent when placing a Hispanic kid.
Caveat: If we're talking about kids who are very young (or, grown up enough not to care anymore...), I'd guess that this factor kind of disappears, since how are they going to know either way? A two-year-old probably hasn't soaked up enough of his mother's "culture" to be "shocked" by another, or at least, to be shocked any more than he already is by being apart from his mother for whatever reason.
And I'm sure that at least some, perhaps most of the people pushing the "it's better for Hispanic kids to be placed with Hispanic foster parents" line would disagree with me about that, they'd say "no, even for two-year-olds, even for babies, they should have a Hispanic foster parent!!!". Otherwise they're "losing their culture"!!! In other words, I'm pretty sure a lot of the social-worker-type people who make these decisions, view "culture" in the usual lefty sort of way, as a race-determined absolute to be preserved for its own sake, especially if it's nonwhite (for all the usual multi-culti reasons). And, I certainly don't. On that I would part company with them, if that's what they're saying.
What about all the white people that go to China and bring back the little Chinese girls? I'm all for it. But, the same people that want only hispanics raising hispanic children will applaud the whites going to China.