The majority of motorcycles are not very noisy in stock form. The issue is with absurdly loud aftermarket exhausts.

So increasing registration tax based on noise level won't make much of a difference. You have to go after the aftermarket stuff.

I've had a really noisy bike (loud exhaust installed by previous owner) and a completely stock bike after that. I vastly prefer the stock exhausts. Quiet most of the time, made a nice sound when I really gave it the beans, but nothing excessive.