The copilot is supposed to take over when the pilot is unable to do his job. It’s Alanis Morissette irony. Like rain on your wedding day or something.

This 1984 panel was made before the days of disposable contact lens. If your hard len fell, you would try to find it (and put it back) instead of just open a new disposable soft lens pack. It was a trope in movies, someone lost a len and try to find in on the ground, someone else step on it or the first person bump into the other person. If both pilots can’t see, well…

I think you’re asking about what is literally ironic in the comic. I think people confuse irony and coincidence very often (as is probably the case in the wording of the character in this comic), and the humor here is that they’re laughing about the silly coincidence of losing their lenses at the same time, but while nobody is left to safely operate the plane.