Goya's use of mirrors in his artwork is multifaceted. First of all, the mirror is a fundamental tool of the trade of a professional portrait painter. It can be used to check compositional elements of an artwork (especially symmetry), but is also a tool by which the painter creates self-portraits, a necessary practice whereby the artist can perfect the ability to capture lighting, colors, and of course a likeness correctly, such as Goya's Self-Portrait before an Easel (6).
But Goya employs the mirror as an element of itself within his images. For example, in the portrait of Count Floridablanca (4) Goya himself is in the portrait, holding up a painting for the Count to inspect, who is also concernedly looking at the viewer, who is in the place of an apparent mirror whereby the Count is seeing himself and is probably checking the quality of the likeness Goya has painted. This is similar to the effect Goya used in the Family of Carlos IV which was painted the same year as the Floridablanca portrait. Again the viewer of the finished artwork stands in the place of a mirror into which the royal family stares at themselves. Is this simply a painterly special-effect, or a comment upon the subjects themselves, of which Goya is a member (after all, he appears as a less important member of the cast of each of these two paintings)?
In the etching Woman before a Mirror with Scythe and Serpent (1), like Frog (2), the use of the mirror is plainer. Each of these two images are judgments made by the mirror which shows the true nature of the subject. These are satirical drawings which assess something directly about the subject. Different is Las Viejas (5) (sometimes called Que Tal? or Time) which, like Caprichos plate #55 (3) uses a mirror as a prop in a morality tale, in each case the elderly person peering into the mirror cannot actually see themselves. The onlooker (such as the laughing men around the woman in Caprichos #55) and the viewer of the painting Viejas see "the truth" which is not grasped by the person actually wielding the mirror.
Goya (particularly in the Caprichos etching series) gave a great deal of thought on how to expose truth by using visual images. Certainly his own moral vision provided the judgment calls he made about how he presented a certain image, whether respectfully (such as Count Floridablanca) or with derision (such as Caprichos #55, which is sometimes said to be a satirical portrait of Queen Maria Luisa). In either case Goya used his intelligence to create images which required reflection on the part of the audience to understand them.