You can do what you want, but remember that you cannot pass any part of the signal through the mixer twice, without the amplification becoming unstable.
Therefore as your system is now, you cannot plug the output of the PC into one of the mixer channels and listen to the mixer output via headphones (because the mic signal will be passing through the mixer twice).
To realise what you want, you must plug the mic and PC as inputs to the mixer and then listen to both from the mixer output via headphones.
Although the above may not be what you want, since the mic signal is now not being seen (input) to the PC.