It might get fizzy, but you won't explode or anything, I wouldn't think. I don't think it will geyser nearly as much due to the amount you can fit in your mouth, plus irregular mouth shape and non-constant pressures vs surface area... (which will mean the Mentos would be effecting less carbonation at a given time)... there is also no 'bottle neck' to concentrate escaping pressures.
I have been looking to see if anyone has done this... it could range between almost nothing happening to a slight foam that wants to come out... but I don't think you would be spewing Coke like what happens in the videos.
Oh, and by the way... I wouldn't drink it from a 2 liter while sealing your mouth over it if you have a Mentos in your mouth. It probably isn't that dangerous (because you can break the seal) but it will cause a reaction between the mentos in your mouth and the Coke which is still in the bottle.