If earth were the size of a marble would it be smoother than a marble? As the title states, If earth were the size of a marble would it be smoother than a marble?
No.

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[2] 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...)