The biggest difference between ground fighting and stand up fighting is the position of the ground. I know this seems obvious but it is really hard to travel through a circular path with your back flat on the ground, it is really hard to pull something toward your hara when your opponents body and the ground are on opposite sides of it. Most aikido movements require too much movement to be very usefull on the ground. The locks still apply they are just much harder to use, unless you are behind your opponent on top of him, in which case, who cares.
I speak here of ikkyo, nikkyo, sonkyo, and yonkyo. Even if you were in a mounted position you would end up unbalancing yourself to get the proper hand positioning. Or just getting beaten senseless while you were trying to.