What is ironic about the laputans’ great expertise in theoretical matters?

Swift's novel was funny in certain ways, but it also had a deep insight in human nature and was also critical and satirical of features.
Laputans were very preoccupied with theoretical matters; they lived literally on a castle in the sky. However, they could not help themselves witht he most basic needs; they could not keep u a conversation because they would forget that they were talking and they got their resources from the people on the ground.
In its unique satirical way, this is Swift's critique to theoriticians of the time; no theory can help you if you do not implement its results, however smart and useful it might be.