State laws and glass breakage coverage differ so you would need to check over your own policy to see if side mirrors are covered or not. This coverage typically applies to all auto glass including windshields, side glass, back glass, door glass, mirrors, sun and moon roofs, and glass-like lenses.
In all states Comprehensive coverage covers windshields and other glass breakage. The terms of Comprehensive coverage may differ from insurance company to insurance company so you would need to read through the terms of your policy however Comprehensive coverage is what covers glass breakage, including the windshield and usually side mirrors.
In some states the deductible for windshield repairs or replacements is waived or you may buy glass breakage insurance so that there is no deductible for windshield damages. Sometimes the waiving of the deductible with glass breakage coverage also extends to other types of glass on the vehicle such as mirrors. Once again to see what coverage your policy allows you or if you are shopping for insurance ask specifically about this issue.
If you have bought glass breakage coverage where the deductible does not apply to window glass damage you will need to read through your policy and speak to your agent to see if the waiving of the deductible extends to side mirrors or if you would need to pay your Comprehensive deductible amount to get side mirror glass repaired.