To start with, it's almost impossible to over grease a BB. It could literally be packed in grease and would work just fine. Too much may slow it down a tiny bit, but really, riding it you wouldn't notice. Grease is simply too viscous to cause a lot of friction. It's much more likely it's over tightened or has damaged bearings. Also, with some cheap sealed bearings there could be a short "wear-in" period during which it may not spin freely, but will act looser over time. Depending on the type of Bottom Bracket, you really could be having any number of smaller problems, but I would start with bearing tension(if it's a loose ball BB), or feel if the bearings themselves spin freely without the crank spindle installed(if it's a sealed system). Or if the spindle spins smoothly(if it's a cartridge like a square taper or Splined), and go from there.