Buzzing heard when using acoustic pickup

Any pickup added to an acoustic guitar will make it sound less like an acoustic guitar. Sometimes it can be a useful sound - IF that's the sound you're after, but don't consider it an equivalent substitute for a mic. The sounds are totally different.

As a guitarist myself, the only time I use a pickup on acoustics is when I'm first writing the song as a scratch track. Otherwise, I always use a mic (with rare exceptions).

The buzz may be due to an impedance mismatch, a grounding issue (bad cable usually), or an external source of the buzzing (such as flourescent lights or a dimmer switch). Turn the dimmer switch and/or flourescent light off and see if that makes a difference.

Make sure you're using an instrument input in the mixer. A "regular" line-level input will give you a weak and frequency-limited signal. If there is no instrument input, then you will need to purchase a direct box. Very usable ones start at about $40 and can cost much more for pristine direct boxes. You plug the guitar into the direct box and take a microphone cable from there to the mixer.