We have a section in the FAQ that states our policy:

We prefer to see projects that have no advertisements but we realize that this isn’t possible for all developers. If you have advertisements, please don’t make them obtrusive or misleading. Avoid sponsored installers, or ads that prevent someone from playing the game without performing some action.

Please avoid putting third-party login walls in front of your game unless necessary. It’s okay if you need to implement DRM, or save user settings on your server. Avoid doing it for the sake of collecting personal information about a player, or trying to redirect traffic to a third-party service.

Any projects that contain malware, spyware or adware are prohibited and will be banned from our platform. This includes any installers that put advertisements on the user’s operating system or browser.

So it's not forbidden, but it's discouraged, and that might explain why few creators do it.