The plural form of cupful is cupfuls and cupsful? Shouldn't we be pluralizing the noun (cup) instead of the adjective?
Cupful is a noun and follows the normal pluralisation rule, so the plural is cupfuls.
That said, cupsful does appear as an acceptable alternative in US English dictionaries https://www.merriam-webster.com/dictionary/cupsful
And the UK English Cambridge dictionary lists it as a US English alternative https://dictionary.cambridge.org/dictionary/english/cupsful
Obviously cupful is a compound of cup and full, so when separating the words you still pluralise the noun, e.g.: 'two cups full'
So this is an interesting quirk of English.
At one point, "cupful" would've been written as two separate words (e.g. two cups full of water). As the phrase grew more common, people started to slur a little and smush it into one word -- cupful. Right as this linguistic smushing was taking place, people still remembered that the "head" of this word, the noun component that determines the bulk of the word's meaning, was "cup." So, people pluralized it as cupsful as a nod to cup having been the actual noun.
However, now that cupful as one word has been in the language for a while, people have forgotten that cupful was condensed from the phrase a cup full of ___. Cupful is its own entity, so it's treated like its own noun. Therefore, it's pluralized like any old noun: by tacking an s on the end.
An example of a word that's in the process of being smushed is "passerby." Plenty of people still think of it as the phrase "passer by" instead of as one word, so you'll see a hot debate brewing between the pluralization passersby and passerbys. In fact, my browser put a red squiggle under passerbys!
I learned this tidbit and some other cool stuff about linguistics from my boy Steven Pinker. This I learned in this book: https://stevenpinker.com/publications/words-and-rules , but I would highly recommend all of his books.
Hope this helped!