Good idea
I think it is a good idea but it could be carried out better. By having less mushrooms falling at a time or else slowing down the speed of the falling mushrooms you could give the user an opportunity to actually catch nearly all the mushrooms, thereby making him feel bad when he misses one.
Also mushrooms with different power ups could fall down, maybe one that turns things slow motion, one that increases mario so he has a larger surface area to catch mushrooms ext. There could also be bad mushrooms, for example one that makes everything go all psychedelic and makes it hard to see the other mushrooms, or one that slows down mario. These mushrooms could differentiate themselves by use of color.
Giving mario the ability to jump could also add to the fun.
Oh and enemies could fall that he would have to avoid or he would faid the level!