Is the thickness of glass tile important in pool installations? If so, would thicker be better?
I think that the thicker the tile in almost any installation would keep it from popping off more.  You would have thinset and grout that have more lateral surface area.  So yes I would say it matters.  How much?  Not really sure.  If you adhere to all installation procedures for your tile it will work.  Maybe the thicker tile has less long-term upkeep (due to the fact that a slight grout failure due to erosion would be less likely).

I will mention too about glass in pools.  It is very difficult to work with.  Most manufacturers require expansion gaps.  Also most glass products at big box stores are not really acceptable for pool installation.   It is actually very difficult to shop for unless you go to a specialty store since it is really impossible to test.  I would say the bigger your pool and the greater the temperature range in your area could both lead to issues with a glass installation down the road.

Not saying that you shouldn't use it but will say that it is a very high-end install for a pool and must be done right or can be very very very costly.