Seems rather niche, for non-key-value-pair data structures (aren't no-sql databases good for that?), considering that lightweight markup fulfills that role for readable document source.
S-expressions are basically directly writing the AST a compiler would normally generate. They can be extremely flexible. M-expressions were supposed to be programming part of Lisp, and S-expressions the data part. Lisp programmers noticed that code is just another kind of data to be manipulated and then only used S-expressions.
Logo is arguably a Lisp with M-expressions. But whatever niche Logo had is taken by Python now.