good is an adjective, not an adverb, so "cooks good" is improper English -- even if "people say it all the time" ;)
Buen(o/a) = Good = Adjective
Bien = Well = Adverb
Adjectives are given to nouns, and adverbs are given to verbs. In Spanish, as in English.
There's 70 million British English speakers in the world but yet it doesn't accept mum.
Why is it only ever women who are good at cooking on Duolingo, wheres the men are bad at cooking..??
Duolingo does a poor job of recognizing appropriate translations. "My mother is a good cook," for instance, should be acceptable. Word by word translation helps build vocabulary, but does little to aid in actual comprehension in my opinion.