In the end credit scene to Ant-Man and the Wasp what happened to Doctor Pym, along with his wife and daughter, when Scott is about to leave the quantum realm? Is Thanos responsible for that?
Yes. They disappeared because of Thanos' snap.
Someone on Youtube posted a nice montage of the end credits of Antman and the Wasp and the scenes from Avengers: Infinity War where characters are turned to dust:
In the montage, you can see that the dusting effect is the same.