It's double entendre. The "face value" meaning is that the disguise should be convincing on Kirk. The "under the surface" meaning is a humorous implication about Kirk's command style. Whether or not Spock meant for it to be double entendre is debatable, but clearly Kirk recognized the "under the surface" implication. If you put the same line in Data's mouth, we'd probably assume that the double entendre was unintended..."out of the mouths of babes".