Having pushed the Jesus Fish symbol into the realm of bumper stickers, Christians have made the symbol as overexposed and overused as much as the cross.
Joining the "Lord's Gym" t-shirts and the "WARNING: This vehicle will be uninhabited in the case of Rapture" bumper stickers, Christians have taken everything sacred and ritualistic about their faith and turned them into mass-market kitsch in the name of popular culture. Having done so, they have no place to complain about the Darwin, Vampire, Linux, and Cthulu makeovers of the fish decals.
They wanted their niche in culture, their grab, their way to say "hey, we're just as cool, and we even have our own outfits"...whether to compete, or to fit in, or a variety of other reasons, it's unsure. But they did. And upon entering the field, they also agreed, by silent pact, to be equally mocked and drawn upon as a cultural resource for kitsch by the rest of society.
If you don't want a powerful religious icon to be mocked, then you shouldn't use it in a way yourselves which lowers the very meaning of it. Popularizing anything makes it trivial, tradeable, and eventually, as with all things, forgettable.
As an added note, the term Christian has become just as overused and generic as the fish itself, being descriptive of both multiple religions, as well as often White Americans themselves.