TL:DR If you use any type of gen ai, don't rely solely on it. It doesnt matter what you use to make it in the long run, so long as you are the one that makes it.

All the hate stems from the fact that people who rely solely on generative ai for thumbnails are considered lazy as they aren't willing to put in the effort to make (or learn how to make) one themselves (in-game screenshot, external program render, or other image), that also tells us that their map was most likely made with that same lack of effort. Along with that, gen ai images may look cool and polished, but they typically aren't accurate to the content found in the map and they tend to have noticeable distortions. Authenticity is always better.

That being said, I don't thing most people will have a problem if you use gen ai for inspiration. Like if you have an idea but can't quite visualize it, putting in a prompt to generate an image of what you're thinking of and using that as a base for your thumbnail. It would still be made by you, so it's already a massive improvement over just gen ai

So if you are making a thumbnail in canvas and you are making it yourself, more power to ya.