Chip-scale frequency comb generators lend themselves as multi-wavelength light sources in highly scalable wavelength-division multiplexing (WDM) transmitters and coherent receivers. Among different options, quantum-dash (QD) mode-locked laser diodes (MLLD) stand out due to their compactness and simple operation along with the ability to provide a flat and broadband comb spectrum with dozens of equally spaced optical tones. However, the devices suffer from strong phase noise, which impairs transmission performance of coherent links, in particular when higher-order modulation formats are to be used. Here we exploit coherent feedback from an external cavity to drastically reduce the phase noise of QD-MLLD tones, thereby greatly improving the transmission performance. In our experiments, we demonstrate 32QAM WDM transmission on 60 carriers derived from a single QD-MLLD, leading to an aggregate line rate (net data rate) of 12 Tbit/s (11.215 Tbit/s) at a net spectral efficiency (SE) of 7.5 bit/s/Hz. To the best of our knowledge, this is the first time that a QD-MLLD optical frequency comb has been used to transmit an optical 32QAM signal. Based on our experimental findings, we perform simulations that show that feedback-stabilized QD-MLLD should also support 64QAM transmission with a performance close to the theoretical optimum across a wide range of technically relevant symbol rates.