It would require the camera to be on without notifying you, which isn't possible on modern OSes. Also, changes in pupil size are caused by many different things. That question is similar to asking whether heart rate can be used as a signal. They're not tracking eye dialation for funny face filters. That level of tracking requires cameras better than phone selfie ones.

"The responses can have a variety of causes, from an involuntary reflex reaction to exposure or inexposure to light—in low light conditions a dilated pupil lets more light into the eye—or it may indicate interest in the subject of attention or arousal, sexual stimulation,[3] uncertainty,[4] decision conflict,[5] errors,[6] physical activity[7] or increasing cognitive load[8] or demand."

https://en.wikipedia.org/wiki/Pupillary_response

https://www.microsoft.com/en-us/research/wp-content/uploads/...