Your first step, however, must be to use either the TimeWarp tool, or the function “Merge Tempo from Tapping” (or a combination of both 
), so that the Cubase grid will eventually match up with your improvisations. No amount of quantizing can sort out the score legibility problems if the grid (i.e. bars and beats) doesn’t line up with what you actually played. Once you have done that (there’s plenty of documentation about those two functions, and many additional posts in here), you can then use Cubase’s Score Display Quantize functions to clean up the way it looks .