The headphone jack is stereo.
Mobile phones would use an external audio codec (e.g. Wolfson), which is very high quality.
Raspberry Pi uses PWM on a GPIO pin with some filtering. It's not so high quality.
If you are playing through speakers built into the monitor, or small powered PC speakers, then it will be fine.
If you have an expensive amplifier/speaker set up, then I'd really recommend using the HDMI audio.