How to fix a baked taco casserole that is too dry

Could be served cold, room temp or warmed again.
Add some chopped vegetables (sweet pepper, fresh tomatoes), sour cream or - as recommended - salsa. In a pinch, a bit of broth.

If you just want more moisture, you could serve it with a salsa. Cooks use the word "dry" in some pretty peculiar ways, though.