Excess grease separating the metal surfaces more than the minimum necessary to exclude air gaps will only degrade conductivity, increasing the risk of overheating. It should also be noted that silver-based thermal grease can also be slightly electrically conductive. If excess were to flow onto the circuits, it could cause a short circuit.
Soak a q-tip in 98-99% alchohol and carefully swivel it around in the thermal paste to get it off, if there is a lot of it you can remove the majority using a lint free cloth such as a glasses cleaning cloth. lint free assures you aren't getting foreign compounds or oils anywhere, and remember, most thermal pastes are slightly conductive and could short circuit the board, even if it's not conductive, it could leave a smell when the unit is turned on. If you don't have that high % of alcohol you can use 90% isopropyl bought at any drug store.