Exceptionally high quality deepfakes in Kendrick Lamar music video
youtube.com
youtube.com
You can feely sing about McDonald's hamburgers, dress as Ronald McDonald in a music video, or do both at the same time.
Permission only comes into the equation when there's a likelihood of a party believing they're conducting business with a party which they are not (fraud by misrepresentation).
To put it more simply; selling burgers dressed as Ronald McDonald is a bad idea. Selling a song under the name "Eminem" while deepfaking Eminem is a similarly bad idea. Using someone's likeness artistically is, by default, absolutely fine.
Aside from "likelihood of a party believing they're conducting business with a party with which they are not", are there any other "rules of thumb" when it comes to this?
What if the owner of Ronald McDonald trademark (apparently, his costume clown face persona+multiple variations of the name are trademarked) didn't like the way the character was portrayed by you or just didn't want you to use it for whichever other arbitrary reason, does their position stand any legal ground? And does their specific reason even matter at all when it comes to this?
Incidentally, this is why Apple Records, Apple Socks, and Apple Computer can all simultaneously exist.
To this day, Apple Computer pays a licensing fee to Apple Records for the existence of iTunes; because Apple Records has retained the trademark for the use of the word Apple in reference to music sales since before Steve Jobs ever touched a computer.