For example, Moby Dick probably has an infinite range of supportable interpretations, but if I said it's a book about selling cotton candy on the moon then I am objectively wrong.
Not really. If you say that it's a book about selling cotton candy on the moon, you are wrong because you don't really believe that yourself, not because it can't be a book about selling cotton candy on the moon. Perhaps someone has had a deeply emotional experience while selling cotton candy on the moon, and Moby Dick reminds him of his cotton candy selling adventure so much that he interprets the story in the light of his experiences — I would say that it's a perfectly valid interpretation.
Here's a quick question: if someone's interpretation or "reading" relies on a mistake (for example, they mishear the lyrics to a song or only watch the first half of a movie) is their interpretation not a "misreading"?
Now here's my longer response that most likely should be ignored, but I teach literary analysis so I can't not say it:
Likely this is a case of me operating under a different definition of "interpretation" than you are. I would say what you describe is a valid
response to Moby Dick but not a valid
interpretation. For me, and I would argue for most critics of various arts, interpretation involves translating the meaning of a text so that other people can recognize that meaning too. Just think of the other ways "interpreter" are used in other contexts, for example a sign-language interpreter. Also, the etymology of "interpret" suggests this same reliance on communication to others. So an interpretation operates as a mediation between a "text" and other "readers."
As a result, interpretations that rely on idiosyncratic and personal experiences are not useful interpretations. An interpretation must point to objective details about a work (its "textual" or formal elements, its context, its relationship to other texts, etc.) in order to be valid.
A response, like the one this hypothetical cotton candy salesperson has, can be valuable because they can point readers in the direction of valid interpretations, but are not valid interpretations in and of themselves. So maybe after considering the connections between Moby Dick and their experience, the cotton candy reader recognizes what the book says about capitalism or living in a strange and alien world. In that case, they can claim that Moby Dick is about capitalism and thus is relevant to selling cotton candy on the moon. But that doesn't mean that the book is about cotton candy on the moon.
