That's literally the entire Winx fandom! Almost every Winx artist has used a Winx base and drew over it with their OC or a canon character from the show.
You could've told them how to become a better artist, like learn how to draw the Winx girls' proportions traditionally than being rude and saying their art is bad. Obviously, their art skills aren't the best but they are being creative and starting their progress to becoming an actual artist.
Before coming after this person, check yourself first