What do you mean "no"? The very laws of physics dictact that it will fail as a weapon
Firstly, it would require one to hold a blade more commonly used for hands in ones mouth. Then, the only way I can see attacking with it is by swinging ones head around. That just won't work, especially not sideways
You'd have better luck with using greaves as a weapon. And they're legplates. Hell, an anchor or yo-yo would be a more effective weapon. At least they would be able to HIT the opponent
I'm sorry but I felt my reality had to come into this. Your request just fails to make much sense at all to me. A sword in the mouth is not a viable weapon in any combat situation I can think of due to the head and neck's limited capability of turning. Now, if you were some creature with an extremely flexable neck, then I could understand using a sword in the mouth. Otherwise, it just fails miserably and is an obvious target for punching in the face