Actually, Finn the Human is a great example of what I mean! While not necessarily "anatomically accurate," he does have some very firmly established design notes that you can spot watching the series. A lot of what makes Adventure Time look good despite its very loose noodly art style is the design documents that swarm around how every character looks and moves!
As you can see, a LOT of thought is put into how the characters move and how their body does/does not contort. Later seasons even give Finn a bit more of a pot belly, to break up his pill body-shape some,
This is them essentially "designing their anatomy," it doesn't necessarily conform to realistic anatomy most times but is still informed by/based around similar rulesets. They have SPECIFIC ways of moving to keep things looking nice.
Essentially, they "built" a very simple skeleton for finn's design that they work from to maintain design consistency.
You can also see this in many other cartoons.
I feel like I made my point here very poorly, the problem isn't that yours isn't anatomically correct (as you ARE going for a cartoon art style) but that you don't seem to have "designed their anatomy" enough for every line to feel intentional, hence me saying it feels like an excuse. I came off far more hostile/elitist than I intended here, and for that I apologize.
