The "Crabs" game can be legal if it is seen as a form of parody. That is one of the forms of "fair use" that allows copying (within limits). This is to allow the use of limited amounts of copying for critical or "mocking" pieces, which are considered a form of free speech.
Two other issues come into play under "fair use." The first is whether or not this is "commercial" (yes) or non-commercial (e.g educational) use. That is mildly negative for "Crabs" but by no means dispositive. The second is the likely market impact, whether the new use tends to compete with the old use in its "home" market, or whether it is likely to open a new market of a very different, perhaps "opposite" audience that might later buy the original as a "crossover."
The "Crabs" game seems to address the "green" or at least "pro animal" (PETA) market. If the defendant can show that the "Cards" market addresses e..g., your "inner Nazi," making it "opposite," that would be ideal. It would be less convincing if "Cards" were addressing e.g. human rights, because that might be seen to overlap with the green market in terms of social conscience.