I'm in the U.S., specifically Oklahoma, which is generally considered part of the U.S. "Bible Belt." It's a fair guess that the vandal was a nominal Christian (thank you for reminding me of that term, by the way, it would seem to apply) due to the fact that the Darwin fish is a defilement or perversion of the "Jesus fish" emblems (sported by modern Christians) which themselves echo the ichthys symbol supposedly used by early Christians. Jews, Muslims, and other religious non-Christians, to my knowledge, would have no direct interest in vandalising a symbol which itself mocks a decidedly Christian symbol. Still just a guess, but like you said, statistically unlikely it would be other than a Christian. Then again, maybe it was a false-flag operation undertaken by Discordians--that thought makes me smile. :-)
Anyway, whoever they were, they broke mine's little footsies off! It made me sad and angered, but life goes on.
Enjoy yours, om, and protect it from mean people! :-)