I don't necessarily disagree with the commentary that putting a cross emblazoned cracker on top of a burger is in poor taste. Personally, I would agree with such statements for a more specific and technical reason. It is literally tasteless. The cracker is without flavor. As for the insistence, especially among Catholics, that it is "crass and offensive" I find it ludicrous and rather hypocritical.
I say especially Catholics primarily because their own doctrines claim that the eucharist* literally becomes the body and blood of Christ. What is more crass and offensive; creating a burger in homage of a band you happen to like or believing that you are eating the actual body of another person? Even doing this symbolically I find rather creepy and disturbing. So, Christians who is really being rude and offensive? I'll taking poking fun at a patently absurd ritual over knowingly ritualizing cannibalism.
*An actual consecrated wafer and wine rather than a pretend one. Assuming, of course, you don't see the whole ritual as a twisted and perverse fantasy.