The short answer is no, cut grass clippings do not contribute to thatch in your lawn. Thatch is a layer of organic matter (living and dead) that accumulates between living, growing vegetation (grass) and the soil.  As long as the thatch is less than about a half inch, your lawn is fine. If it gets above that level, depending on your grass type, you may want to consider dethatching or aerating (which will inevitably remove some of the thatch). (Note: Aeration is when you pull plugs from the soil to help air, water, and nutrients circulate better throughout. This helps roots grow deeper and stronger, and you have a greener, healthier lawn as a result.)