Honestly, your best bet is to talk to a DAME before booking your medical - Even ask if you can have an appointment to discuss the medication prior to booking a medical (then it doesn't show as a refused or cancelled medical). CASA can be a bit iffy with any medication linked to depression and also with sleep issues - so if they are ok with it you are likely going to have to jump through some hoops to get your class 2 issued.
Unfortunately, there is nothing listed on the CASA DAME Clinical Practice guidelines page indicating that the use of Mirtazapine is ok or not.