As the title states, If earth were the size of a marble would it be smoother than a marble?
This What-If page describes the case for 'smoother than a bowling ball"
These scans (along with various measurements of ball roughness1 tell us that a high-end bowling ball is quite smooth. If blown up to the scale of the Earth, the ridges and bumps would be between 10 and 200 meters high, and the peaks would be between one and three kilometers apart:
By Earth standards, this is quite smooth; our highest mountains are 40 times higher.
And it is simple to continue the scaling process to show that the same holds true for marbles. (Although I have not found a good source of data from people who scan marbles...)