Not everything is a mantle. There's no reason to suspect dragons are.
Given the Cosmic nature of
Dragons, I'd be quite suprised if they are not in fact mantles. Especially, given the WOJ that each Eldest of a given race is a mantle (in Fae at least). And given that they CREATED an entire servator race of the lesser
dragons, if Dragon's aren't Mantles, then they are probably far and away the most Powerful thing kicking around that isnt.
Didnt michael kill a dragon with his sword? That might mean it is a mantle killer item.
No it doesnt. Harry has killed two mantled Immortal's so far, and has yet to Destroy a Mantle. For all we know that dragon's Mantle is sitting in a Dragon Egg somewhere waiting to be Reborn, or got even distributed to the two surviving Dragons. Or something. We only know how the Fae Queen's mantles are passed, and they are unique enough overall that I dont think they are going to be the best example of "typical" mantle behavior.