I only have mashed potatoes on Thanksgiving (I mean I eat them year round, but that is the only kind I serve on Thanksgiving). Maybe it is a Florida thing? I also had never seen anyone serve macaroni and cheese with Thanksgiving dinner until I moved to South Carolina. But everyone I know here does. They also serve rice and gravy instead of mashed potatoes and gravy. Rice seems to be a bigger part of the diet here than potatoes overall. Of course they used to grow rice here, so I guess it makes sense.
We had potato salad one Thankgiving because we did a potluck. It worked for us, so my Grandma did not spend all day in the kitchen. If we had the traditional type dinner she would have spent all day in the kitchen. This was in Kansas where we had the potato salad for Thanksgiving.
Mac and cheese seems like in everyday dish for Thanksgiving, but I guess so are mashed potatoes.