Not necessarily - it depends what you do with the FFT output. If you threshold it/quantize it/throw something away, then yes it is.
Alternatively, if you preserve *all* of the phase/frequency output data, and use it to reconstruct your original signal via the IFFT, then no it's not.
The FFT is simply a transform, and doesn't change the overall signal (information) entropy - it just moves it around. It's not lossy unless you start throwing information away.