Yes, it is a definite side effect. I was on mirtazapine 30 mg. years ago & at that time a life savior for me.
I had suffered a very traumatic event and could not sleep or eat (or do anything really). However I was not hospitalized. After lots of trial and error with other SSRIs it was this drug - a SNRI - that help me sleep and gave me an appetite. However, be aware...this drug can/will caused major weight gain along with increased lipid profile and triglycerides. Eventually I had to go on the statins, my cholesterol, etc. became so high. But I needed this medication at this time in my life - it was the only medication that returned me to a "normal" life and I was able to go back to work, etc. & become fully functional again.
I took it at 9 PM nightly. In time, if you are on it for "long" term your body will eventually adjust to it and your fatigue will not be so pronounced.
Hope it is helping with your depression. All the best!