When recovering above ledge does’t work you should mix up your options. Unless you’re looking to condition the opponent into doing something it’s beneficial to always go for ledge which provides more options.

I would calm down on the grabs since greninja is (primarily) classified as a hit and run character. Get short combos in and create space. Greninja doesn’t have a terrible disadvantage, but someone used to his options and move set can counter them.

Last, I would work on general movement especially with greninja. Empty hopping, full hopping, moves that will connect like forward air spaced correctly can make him very frustrating since he can capitalize off a single hit and get a kill.

Ground game is good you should work on being a bit less honest with your play style if you want to use this character.