There is a vernacular construction talking about the thingness of various topics of popular conversation, as in:
Is/are X really a thing? = Is there truly a current trend about X? Is X popular right now?
Is/are X still a thing? = Hasn't the hype about X died down?
Why isn't/aren't X a thing? = I think X is cool and everyone should get excited about it!
And, of course, How come? is a slangy term for Why?.
In all of these cases, the slang term is a thing, which is distinctly singular. This vernacular construction is almost never things. (Based on google searches for several constructions.)
That means that any time you're talking about a plural object, the grammar gets tortured a little bit. To a native speaker, the abuse of the language tends to be more painful (and thus, correspondingly less common) if the subject and verb don't agree, and if a plural noun is right next to the word a. Modifiers like really and still act as buffers so that our internal critics don't scream at us as loudly. ;-)
How come "pumpkin noodles" is not a thing?
With quotes to indicate you're considering it as a distinct object, it's marginally ok in vernacular speech but still sounds awkward. It's better if you can rephrase it so that the verb agrees, even though we're still calling it a thing.
Why aren't pumpkin noodles a thing?
How come pumpkin noodles aren't a thing?
The contraction sounds better to me than "are not" in this case because it's relaxed, vernacular speech, and because we are not emphasizing the negative. I'd use the contraction unless I wanted to negate it and say I don't like pumpkin noodles:
Pumpkin noodles are SO NOT a thing!
To address some of the comments and other answers, if you change a thing to things you are breaking the vernacular construction and are now questioning the existence of the object (or maybe, whether it is person vs place vs thing) and not questioning its popularity. In the context of the question, it would sound like a clumsy attempt at speaking in vernacular.
By the way, this construction is perfectly fine in normal speech, but would not be acceptable in formal writing or speech.