"Tatty" is an adjective, and almost anything can be tatty, even a Masi.
"Tat" on the other hand is a noun (singular and plural) and implies uselessness and worthlessness.
So evidently a Masi and Tat are mutually exclusive as someone will always put worth on a Masi. especially when they are trying to sell it.
Personally I have never owned a Masi and have no axe to sharpen at the wheel.
Regards
Martin Coopland