This bit is a little outside my experience as I'm still new to making and publishing assets, but as far as I can tell it would all come down to the licenses on anything you intend to use. I previously looked into licensing aspects when I first started thinking about making games and considered using pre-made assets as a shortcut, but there are a lot of different licenses with differing requirements on usage and attribution. If the license allows you to re-use and re-purpose the assets you plan to use, then there shouldn't be a problem, but some assets will require attribution, and others might be free to use personally, but not commercially. This is why I decided to try and make my own assets as much as possible so that I didn't tumble down that rabbit hole.
The only exceptions to this would be adherence to Itch's own policies and the usage of AI generated assets.
Hope this helps.