I was under the impression LISP is nice to write and work with but difficult to read. How well can you understand LISP written by a LLM?

Honestly LLMs write decent enough Lisp. They do however tend to want to use only functions and thus it ends up a bit verbose, but prod them enough and they'll make macros that work and write nice code. Lisp written by LLM is nicer than Ruby or C++ written by an LLM.