It bugs me when people say Cloudflare is a MitM, because that is a disingenuous representation the situation.

No, it is a clear description of what is happening: Instead of https keeping the traffic encrypted from user to service, it runs only from user to Cloudflare (and then in some cases from Cloudflare to service, although that's irrelevant here). The result is that a third party (Cloudflare) is able to read and/or modify the traffic between the two endpoints. This is exactly what we in mean in cryptography discussions by man-in-the-middle.

You can decide that you don't mind it because it's not a secret, or because they haven't been caught abusing it yet, but to say it's not a man-in-the-middle is utter nonsense.

and you opt into it.

No, the service operator opts in to it, without consulting the user, and usually without informing them. The user has no choice in the matter, and typically no knowledge of it when they send and receive potentially sensitive information. They only way they find out that Cloudflare is involved is if Cloudflare happens to generate an error page, or if they are technically inclined enough to manually resolve the domain name of the service and look up the owner of the net block. The vast majority of users don't even know how to do this, of course, and so are completely unaware.

All the while, the user's browser shows "https" and a lock icon, assuring the user that their communication is protected.

And even if they were aware, most users would still have no idea what Cloudflare's position as a middleman means with respect to their privacy, especially with how many widely used services operate with it.

To be clear, this lack of disclosure is not what makes it a man in the middle. It is an additional problem.

it cannot be a MitM because both sides of the connection are aware of this layer.

This is false. Being aware of a man in the middle and/or willingly accepting it does not mean it ceases to exist. It just means it's not a man-in-the-middle attack.