I have used this on pianos also, but I find a regular mic works a little easier on a piano.

I use this on a grand piano as a remote mic for a Korg chromatic tuner and it works well.

I recommend getting a contact mic

The contact mic works well with my Krog tuner

I use this mic with my Korg tuner and really like it

In fact, I have stopped using it because the internal mic seems to work fine for my purposes (piano tuning).

You are also probably frustrated because your electronic tuner's mic picks up ambient noise and makes it difficult to tune

The contact mic works well with my Krog tuner