In general, mirrors should be hung at eye level, using the general guideline that the vertical center of the mirror should hang between 56 and 59 inches above the ground. This rule applies when hanging a mirror over a couch.
It is always better to hang a mirror a little too low than too high. If a mirror is hung too high, people have to strain to see themselves in it, making it feel awkward and uncomfortable. If the couch has an especially low or high back, or if the mirror is an odd shape or size, such that the 56- to 59-inch rule does not seem to apply, then the bottom edge of the mirror frame should hang 5 to 9 inches above the top of the couch.