I have encountered many music instrument apps such as piano, guitar or even some wind instruments mapped onto the touch screen of the iPad. They can be fun to play, but would playing on the iPad and learning to adapt to the touch screen hurt my playing when I go back to the actual instrument?
I would add also that if you replace practicing on a real instrument with tablet practice for an extended period of time, yes, you will notice a decline in your technical ability at the actual instrument.
There’s a limited amount of potential gain from practicing on a virtual instrument as opposed to the real. You could learn to sight read notes, guitar fingerings, etc., but there are some crucial elements of playing any instrument that can only be learned by practicing on the real thing.
So, short answer: use the tablet to (if anything) supplement, not replace.