The name of a tool is often closely associated with its primary use case, not the mechanism of its function. Tools which offer expansions of a particular use case are often named for the tool that they build upon, not for the mechanism by which they fulfill that use. Smartphones, for instance, are clearly computers, yet we call them "phones" because we use them for social connections over long distance—it expands the use case for the Western Electric 500, not so much a Compaq 386. (This is why the fight to preserve total app freedom on Android is a lost cause; success would make an Android phone a better Compaq but a worse phone.)

If the queen had an ordinary mirror, she would use it to ensure and maintain her beauty. It's specifically a magic mirror because it expands that use case, through magical properties that allow her to compare her beauty to that of every other woman in the kingdom.