Why Q is the ratio of a resonant circuit's center frequency to its half-power bandwidth, BW = f0/Q. An impedance-matching network is a resonant circuit, so raising its Q sharpens the response and the range of frequencies over which a good match is maintained shrinks. A low-Q matching network is broadband and covers a whole band; a high-Q network may match only over a few tens of kHz and needs retuning as you move.
Watch out The idea that harmonics increase is backwards: a higher-Q network is more selective and actually attenuates harmonics better, which is why tank circuit Q matters in transmitter output stages.
BW = f0/Q. Q up, bandwidth down. High Q = sharp and narrow, low Q = broad.