You should definitely go to an exit interview.
In the case of a sour breakup, you should 100% lie. Just say it was great but you were in need for a change. Don't dramatize the lying part because you lied to get in and the employer lied about how great their company is. Work is a lie, play the game.
In the case of a drama-free breakup, when pressed for feedback and the room being receptive of it, do share your thoughts. But toned down, in a light hearted manner. Comment on things and processes, not people. This gives the impression "helpful until the end".
Both ways are a clean exit, with no bridges burned. There might come a time in your life where you're less desirable on the market, having intact relations can make a huge difference then.
You should internalize the dishonesty as a type of strength, because that's what it is in this context. In a sour breakup, you're given the perfect opportunity for revenge, with little immediate consequences.
Your ex-boss Tim is an asshole. He knows he is. And you know he is. You now have the opportunity to finally say it to his face.
Applying silence in this situation is powerful. It says so much more. I could harm you right now, but I chose not to. This radiates self control, which is widely respected. Meanwhile, Tim was looking forward to a good burn and argument, but didn't get any. He feels submissive, like a Roman emperor giving him a thumbs up, allowed to live another day.
And that's how you play it. Alternatively, you can be 21 years old and post on Medium "7 reasons why I quite company X after 8 months". Let me know how that goes.