I don't know a whole lot about the Mirena, but I've heard that ovarian cysts are relatively common after having it inserted. I've never used the Mirena or other type of IUD, however, I have had ovarian cysts (I've since had a complete hysterectomy due to other problems). Normally they will just kind of keep an eye on the cyst to see if it is getting larger or if it starts causing you pain or other problems. Sometimes the cysts will go away on their own.
Has your doctor recommended any specific course of action for the cyst? If it is not causing you any problems such as pain or increased bleeding, etc., and your doctor has recommended to just watch it, then I would definitely agree with that. Most of the time, they are nothing to worry about.
I had mine removed from all of the issues including cyst that the mirena caused..Most doc will say it's not the mirena but I didnt have any prob til I was on the mirena!! It may not be bad for you as every woman is different but it was horrid for me and many women on this site can attest that now they have trouble conceiving now that the mirena was removed. Many have had miscarriages because of it as well. Research the mirena and you will see for yourself how many women have issues with it..Again mine exp was hirrble and I will never recommend it to anyone but other women have had nothing but good things to say..if you are already having cyst I'd say remove it