There are a few factors at play that make a language good to learn: simplicity, learning materials, and suitability for learning projects.
On the first point, Fennel is simpler than any other lisp I know, beyond "my first interpreter" type Schemes. But you'll find much better learning materials for some other lisps; in particular Racket is exemplary in this regard.
So I would say it comes down to the learning projects you might want to build; this is always the trickiest part of learning any language. If you want to learn by creating a game, Fennel is a great choice. Likewise if you want to extend an existing program that already has Lua support, such as AwesomeWM or Redis. But for most "industrial" type projects that access external APIs or write to existing file formats, you'll have a better time in Racket due to the better library ecosystem.
I don't find it particular simple or a good Lisp. I don't think it is a Lisp at all. It does not even know about QUOTE or basic list processing. If Fennel is supposed to be a Lisp, than the concept of 'being a Lisp' is meaningless.
Actually the best Lisps are those who are actually Lisp (actually implementing the particular syntax and semantics of Lisp) and not a transpiler to another language. This other language leaks on all places.
Fennel is neither a Lisp nor a good learning environment for Lisp. Fennel is actually some kind of Lisp/parentheses-inspired layer on top of Lua. That's nice but not Lisp.
There are a lot of small Lisps which make learning Lisp much more easier. Start with any small Scheme implementation and that's much nearer to Lisp than Fennel will ever be: s-expressions, list processing, evaluator, code as data, macros, data representation, tail/recursion, procedural abstraction, first class functions, lambda calculus, etc etc. There is a lot of excellent material in the Lisp world to teach that. There is even material how to do useful things with that. None of that applies to Fennel. It does not do list processing, its code as data is weak and Lua leaks everywhere.
If you used Lua before, then this seems like perfect lisp to learn. If you don't know any Lua then some features will confuse you (multiple return values, error handling, arrays as tables, pairs/ipairs) because all these design decisions make for seamless inter-operability between two languages.
Perhaps for learning purposes you would benefit from lisp that has tools with debugging capabilities, like Racket.
