Why is there a weak sound when my microphone is plugged into my instrument input?

The microphone sounds great when plugged into the mike /aux in,( on my amp)
But when I try to put a mike into the instrument input the volume goes way down. Do I need to run it through a preamp or DI box first?

Your instrument input is not expecting a mic-level or line-level signal. It’s expecting an instrument-level signal. The following video will shed some more light on this.