Is the atmosphere becoming thicker?
No, the atmosphere is not becoming thicker.

If anything at all, the atmosphere is getting thinner, but only on very long time scales.  Planet Earth very slowly loses parts of it atmosphere due to atmospheric escape, either to space or to the solid Earth.  Historical atmospheric pressure is hard to determine, but billions of years ago, it might have been much thicker than it is now, in particular with a lot more greenhouse gases.  One clue is the faint young sun paradox; we know there was liquid water at a time the solar output was 20% less, which would require so much greenhouse effect that the total atmospheric pressure must have been much higher.  But the jury is still out on that one.

The atmosphere is not becoming significantly denser, either.

The concentration of gases that we are adding to the atmosphere is typically measured in parts per million (ppm) or parts per billion (ppb).  The effect of adding to this is small compared to the natural variability in atmospheric surface pressure / density due to weather and other effects.

The atmosphere does not act like a lens.