The fact that we have critters now that live in magma should be all the proof we need that surviving in magma is not ridiculous in the world of DF. Knowing that things CAN live there, it becomes only a matter of personal taste what should and shouldn't live there. Looking at what is "realistic", we begin having to consider what the ecosystem of a DF magma layer would be like. Obviously we have creatures that live down there, would those existing creatures need to eat? Even if the answer is yes though, they might not need anything like plant life to eat, but could eat rocks, magma, or other substances. If we do decide that there should be a "magma ecosystem", we begin to need to work out food chains and nutrient cycles.
It does seem to me that it would be unexpected that anything that grows in magma would be edible to dwarves, though it could be possible that after removing a thick heat resistant core they could find something edible. Finding things that could be useful though wouldn't be so hard to believe though, whether as armor, as decoration, or as a trade good. Personally, I don't see any overwhelming NEED for magma crops, but the world would definitely allow for their existence without any real breaking of realism beyond what already exists. Do those who feel magma crops are unbelievable have a similar problem with existing magma life?