Forget grappling, Mayweather would get destroyed on the feet, he has never fought someone with 4oz gloves with elbows, kicks, knees etc
He would not get destroyed on the feet. He's an incredible athlete, with better footwork, reactions, handspeed, accuracy, defense, IQ, and stamina than anyone he'd be facing in the UFC.
As you've no doubt noticed, actual boxing has now surpassed Muay Thai in effectiveness and popularity in the UFC. People are learning to box properly, not going to Thailand to train MT. Fuck, even GSP hired Freddie.
And look at the success people like Conor, Stipe, Cody, Ngannou, Holloway are having in MMA. Every single one of them keeps the fight standing at pretty much all costs, and four of them have a boxing background and really just use basic amateur boxing to easily defeat opponents. I realise Max has more of a MT background, but these days he's essentially a pressure boxer, who just overwhelms opponents with punches, using solid workrate, accuracy, and bodyshots.
Floyd is about 100 times better at boxing than anyone in the UFC, therefore given the right MMA training (primarily TDD and LGD), he'd be very successful.