How can I undo/redo in iOS Notes.app w/o shaking my iPhone?
2019 Update

iOS 13 now supports gestures to easily undo/redo!


Three finger swipe:


right to left - undo 
left to right - redo

Double-tap with three fingers - undo
Three finger tap - open shortcut menu containing buttons to undo, cut, copy, paste, and redo (screenshot below)




More info about the new gestures (including copy, cut, and paste) can be found in this article.

NOTE: These gestures should work across all apps that support undo/redo functionality!

Hat tip to @Jaime Santa Cruz's answer, which prompted this update.



You can now press undo/redo buttons in iOS 11 on the iPhone!

Open notes, view a note in landscape, and you'll see arrows on the bottom left!



NOTE: Seems to only work with notes under an iCloud account