Is it good practice to use diodes in a stereo to mono conversion?
No, it is not good practice for audio, in fact it causes the circuit to not work.
Imagine there are positive peaks on audio. The capacitor would always charge on the positive peak but never discharge on 0V or negative peaks. The capacitor would just slowly accumulate charge from the audio to have a voltage over it, until voltage is enough that it will never make the diode to conduct again until discharged.