Collard greens are really nasty unless cooked right. Most white people I know that would server them would just boil them forever and then serve with a bit of vinegar like spinach. Icky. I have had them in soul food restaurants and liked them very much. I think this is part of the stereotype because many whites would not eat collard green, at least not with enthusiasm.
Fried Chicken has the benefit that it is portable, keeps well and is easy to eat without untensils and tastse good cold or at room temperature. It is a good journey food and a good picnic food and maybe it was more likely that whites would see blacks eating fried chicken. My Uncle used to drive a bus from St. Louis to Chicago and the majority of the passengers were black. And many of them brought homemade fried chicken to eat on the way. Sensible of them. Fried Chicken was not an unsophisticated food, you served it to the parson when he came over to Sunday dinner after all.
Watermelon too because it is a picnic food. Anything eaten outside the home is liable to become part of the stereotype. It is also messy and easy to depict. It is easy to look like a fool eating watermelon. I know I never cared though if it was good watermelon.