What time of year are you thinking of? Time of day? I wouldn't say mockingbirds stand out much in this respect. Around here they're likely to be perched on medium-sized shrubs or brambles, or midway up a tree, in other words either partially concealed or very close to good cover. Same as you'd see with many songbirds, e.g. song sparrow.
In breeding season, cardinals, for example, are much more likely to perch way up high in the open (e.g., atop a spruce) and sing. Male Red-winged blackbirds are also champion singers and like to occupy the most visible perch available in their wetland habitat.
In the evenings, lots of birds roost high - mourning doves spring to mind - but don't sing much while doing so.