It all depends on what you are drinking. If you are buying liquor, wine, champagne, or well drinks then the answer is yes. But if you are buying nonalcoholic beverages like soda or juice then the answer would be no.
It's free to drink beer at Mohegan Sun! You will have to pay for it of course, but there's an outdoor craft brewery called Barrel House that crank out some great beers exclusively for Mohegan Sun visitors. Every day they rotate out one of their five servings which includes Scottish Ale, Black & Tan ESB, Nut Brown Ale and Blood Orange Gose (to mention just a few). The food menu there is also excellent so this seems like great place.