Does Short-time Fourier transform impact quality of signal?
STFT (short-time Fourier Transform) should not impact the quality of the reconstructed signal if properly implemented.
Furthermore, any signal $x[n]$ should be exactly recovered from its STFT analysis coefficients by a proper implementation of the associated inverse STFT;i.e., The forward and inverse pairs should exactly undo each other.
One should keep in mind about the overlap parameter and window functions however;i.e., if the window includes zeros, the inverse may not be possible at some overlaps. If the window does not contain any zeros, then the inverse is always computed.