Well, you nailed it. There's a chain and things are reacting to it.
I want to say, first off, that this is currently the game that I've most wanted to keep dicking around in even after I reach the moment where I realize what it's going for. It's so extremely fun just to go "vrooom" in this one! It reminded me of games such as Donut County or Katamari Damacy.
That being said, there are a lot of things here that are technically not great game design, and tweaking them could take this from a fun "wheee" game to something with more depth. For instance, I actually didn't realize there was a whole shoot/retract mechanic for the chain till after a couple sessions. I was just dragging the chain around and steering it into things. I actually think that's very funny and how I enjoyed the game most, but it does mean that, for me, it's maybe a different game than the one you intended.
I agree with the feedback about progression and rewards and the grapple mechanic being confusing. My first thought regarding the grapple was that if I increased the jump and grapple enough I could quickly reach more areas with more stuff to smash, but it doesn't seem like the player is able to launch the spike ball into a rock face and then pull to it.
I found the mouse capture kind of annoying, but you deactivated it during menus and things, so it wasn't too bad.
One thing I'm not sure anyone's pointed out yet is that Doki seems just as able to smash things with her face as she is with her chain. Arguably she's more able to do it, since the player has more direct control. This is extremely fun, but perhaps could use tweaking so that the player is encouraged to use the chain more.
The seemingly inexplicable hostility towards horses was very funny, but I think there's an opportunity to add more references by including different objects and creatures.
As I said, this is probably the game I enjoyed the most when it comes to the actual gameplay. I think it could be something extremely satisfying if you were to work on it more, and please let me know if you do.