Minor illusion says:
If you create an image of an object - such as a chair, muddy footprints, or a small chest - it must be no larger than a 5-foot cube. The image can't create sound, light, smell, or any other sensory effect. Physical interaction with the object reveals it to be an illusion, because things can pass through it.
You could create an illusory fog cloud in a 5-foot cube. However, things pass through real fog clouds just as easily as through an illusion. So how exactly does "things passing through it" reveal an illusory fog cloud to be illusory?