it's a language that's trivial to implement because it's well designed and simple, it embeds very nicely, and it's fantastic for use as a debug shell and to implement guis. it's a great technician's language, if you work with technically-minded people who aren't necessarily programmers, it's a great way to hand them deep interactive power without the footguns of a forth.
I would say it's cleverly designed. Well designed? Hmm, would a well designed language have such a basic flaw as comments that can only be used in very specific places?
I understand where they came from here: the Scheme-like obsession with purity (the enshrined Endekalogue, now Dodekalogue) didn't mesh very well with traditional comment.
Yeah, Tcl has its design warts, but I don't think it has that many remaining that can't be fixed via metaprogramming. Even the popular Python manages to frustrate me with its idiotic statement/expression divide (they doubled down by making match() a statement...) and constant need to convert between generators/iterables and lists.
Thing is that R6RS Scheme (or R7RS-large if it comes out one day) is basically a better Tcl if you only consider scripting and don't need the event loop. If Tcl had played its cards right, it'd have competed with fish/rc/nushell/powershell instead, it was really ready to be a better shell well before any other.
