this should go without saying but it’s really not good etiquette to comment on someone’s fic or art about how you don’t like the characters presented and would like to see them create works about a different pairing.
as a creator there’s nothing more disheartening than someone coming up to you directly and saying ‘yeah you made this thing but I don’t like this thing you should make this thing that I prefer’. it’s not even constructive criticism, it’s just unkind, and couching it with ‘this is good but…’ doesn’t make it complimentary at all.