Sure, but if it's on the verge of toppling then the normal force acts through the lowest point of the block (A). Its moment about the c.o.m. opposes toppling, so as BvU says there is no moment about the c.o.m. to cause toppling. If you are taking moments about A then bear in mind that a moment is required to account for the acceleration down the plane. Toppling will only occur if the moment that way exceeds that required by the acceleration.Even without friction, the normal force and the weight can produce a non-zero couple when the angle is sufficiently great.
Once sliding, I believe the critical value of ##\mu_s## for toppling is block length / block height.