I cast Extreme slob americanification.
Mettaton will become beyond morbidly obese, with a heavy, hard-to-understand American accent who doesn't care about his hygiene or health, wanting to do basically nothing but eat, and also grill on a massive bbq, all his underwear having the American flag pattern.
Mettaton pulled out a mirror immediately after the spell hit him, worried about what was about to happen. The most immediately noticeable effect was seeing his hair go from black to a very artificial looking dyed blonde. A loud gurgle eminated from his gut, the sound spreading across his body before it BWOOMFED out with pure fat in a matter of seconds. His clothes were torn to ribbons in an instant, all that remained being a pair of stretched out red white and blue briefs. They hugged tight to his double wide flabby butt, much of them being wedged between those swampy cheeks. They were also VERY tight around his new FUPA, soaked in seconds by pre leaking out his buried cock. A scream built within his throat as he dropped his hand mirror but it was replaced by a window jiggling loud belch, joined after by a fart that doubled the room's humidity. His lids lowered into a passive, livestock-like expression as his flatulence petered out. He scratched one of his wide flabby boy tits and spoke "Gawd fuckin' damn Ah need me a beer and some hawt dawgs."