Are you sure it is mites or could it be lice, there is a significant difference.
Mites live in the crevices of the coop and crawl along the roosts and onto the chickens at night and suck their blood, this can cause anaemia and possibly even spread disease, so it is a potential health risk.
Lice live on the chicken and are more of an irritant than a serious health threat. They live off dead skin and dander debris (they don't suck blood) and healthy chickens are usually able to keep them in check by regular dust bathing... unless there is no suitable dust bathing facility available.
Knowing which pest/parasite is the problem is helpful in how you treat it although both can be treated with the same chemicals, obviously the coop needs to be the main target when treating mites, whereas the chickens need to be more of the focus of treatment if the problem is lice.
