Psychological behavior of talking alone in front of mirror.
By itself, a single incident like this, or even a habit of talking to oneself in the mirror, doesn't qualify as a mental disorder, in the opinion of one psychiatrist.
For a bit more background on the research in this area, there's a NYT article. Alas it doesn't get into children... and children to have more behavior that few adults have, like imaginary friends.
So the contents of the external self-talk is probably more important than the mere fact she does it.
The introductory part of an article focused on self-talk in Down syndrome mentions that

Self-talk or talking out loud to oneself (also referred to as 'private speech') is a common behavior in young children and plays an important role in the development of higher-level thinking and self-directional skills. Developmentally, children engage in self-talk to coordinate their actions and thoughts. During focused goal-directed activities, self-talk/private speech systematically becomes more selective in 4-year-old children than in 3 year olds[1]. By the age of 6 or 7 years, most children stop talking out loud to themselves and develop an internal dialogue when working out problems or challenging tasks[2].
In certain situations, people engage in self-talk (either positive or negative) during periods of stress or conflict or to motivate themselves. Negative self-talk has been found to increase during levels of affective stress in children and adolescents with anxiety and depression[3,4]. Studies of the use of self-talk in people engaged in exercise and competitive sports have found that exercisers and athletes use self-talk as a positive, motivating behavior to enhance their performance[5,6,7]. Few studies have been conducted on self-talk behavior in people with intellectual disabilities. Self-talk has been a common observation by parents of children and adults with Down syndrome[8,9]. It was postulated that self-talk is an important aspect of the thought processes of people with Down syndrome when they are working out difficult situations, expressing feelings, and entertaining themselves[10]. No association was found between self-talk/private speech and behavior problems, communication difficulties, or social isolation. However, self-talk in adults with Down syndrome has sometimes been incorrectly considered to be hallucinatory behavior [11,12].