Major and Mrs. Monarch stunt the narrator's work as an artist. Why? Because they're too real. What Major Monarch thinks would be his and his wife's greatest strength as models proves to be the narrator's undoing. He simply can't do anything with them; however he chooses to paint them, they will remain as they are: ordinary, respectable, middle-aged.
Inevitably, this holds back his creativity, making it impossible for him to exercise his imagination to the fullest possible extent. To paint a really good portrait, he needs to have something to work with, something he can mould to create something new and exciting. But he can't do that with the Monarchs. They are who they are and cannot be anyone or anything else. And this is a serious drawback to being an artist's model, especially for an artist like the narrator who's so wedded to artifice.