Can relate to that. I often wear over-sized terry cloth lined trainer past over my disposable diapers at night, especially if I am staying away like at friends houses.
As little Sheri says, they are professionals, will be focussing on your mouth not your torso and you probably won’t be the first to wear a diaper to an appointment. You should go dressed in a way that is comfortable and relieves any anxiety. Nothing worse than having an embarrassing accident in the chair and waking up from sedation with that situation to deal with.
Last time I went to the dentist I wore because I was going under sedation and didn't trust my ability to stay dry at that time. If you have any doubt or it just makes you more comfortable in that situation then definitely wear one.