I agree. People who use the language make the language. Lexicographers are just around to record what happens, not to prescribe usage. Its antecedent "embiggen" pokes fun at the mechanism by which English contrives new words with prefixes and suffixes, often at the expense of a more suitable existing word. As such it shares a bench with the baroque "antidisestablishmentarianism," also perfectly cromulent. Start with a faux root, add an anglicized Latin adjective suffix, give it a context from which its meaning can be inferred, and you have the makings of a new adjective. Since it has an agreed-upon meaning -- "ironically our counterintuitively valid" -- it thus fits a niche. This is how the language grows, intentionally or otherwise. It is as valid as any other word.
I confess that I'm a prescriptionist by nature. However, I would posit that more Americans, at least, recognize "cromulent" than many other words of longer standing. Heh--just as an example, a lot of people know that "antidisestablishmentarianism" is a word, but how many of them actually know what it means? So leaving aside that we all understood in context what "cromulent" meant in its first appearance--Mrs. Krabapple's throwaway usage is probably the funniest part--enough people understand it out of context that even prescriptionists must accept its validity. And there is enough difference between, for example, "cromulent" and "valid" that it serves its purpose as no other word can, and I'm always supportive of words that increase precision.