iPhone 12 is bent and doesn't turn on

Well, I tell ya, that's impressive if nothing else! Not only is it pretty hard to bend those things, but you've done it in two dimensions, not just one! Wow...

Anyway, back to your actual question; yeah, I'm inclined to agree with your assessment; with no signs of life and that much exterior damage it's not looking good. Outside of opening it up and physically examining it, the only other test I can think of would be to try a wireless charger (if you haven't already). That would bypass a damaged charging port since they use a different connector into the motherboard.

Unfortunately the sandwich board design of the motherboard means it cannot tolerate any degree of flexing whatsoever; the interposer (the connector between the top and bottom boards) connections will break very easily if enough force is exerted on the board assembly to flex it. That's mainly where I'd be concerned with your particular situation.

The best way to tell would be to swap the motherboard into a functioning phone, but that's a bit extreme just for a test if you don't happen to have a spare 12 lying around. Otherwise, you could try a new battery and charging port; you can get those for a reasonably minimal amount of money. If changing those two things doesn't get it to show any sign of life, I'd be tempted to write it off at that point. I have resorted to taking an iPhone X to a shop for a similar set of symptoms; getting a diagnosis wasn't too expensive and it did give me the information I needed to decide where to go with that particular issue.

Good luck with it; be sure to come on back and let us know how it all turns out! If nothing else this can be a learning experience for all of us.