There have been many of this type of “crossover” comic strips in recent years, and I’ve always wondered about the legal aspects. Are Lio and Peanuts features offered by the same syndicate? Would that get around copyright issues?
The technicalities are not always intuitive. For example, if a copyright or trademark holder ignores unauthorized use of their protected material, it can be legally regarded as “abandoning” the trademark or copyright. For example, if a children’s charity created a clown spokescharacter called Donald McRonald, people would probably be mad at McDonald’s if they sued the charity for copyright violation. But if McDonald’s simply ignored such usage, they’d put themselves in jeopardy of losing their legal ownership of their own spokesclown, Ronald McDonald.
Otherhandwise, if a cartoon wrongly implied immoral behavior to a well known public figure, the real person probably couldn’t successfully sue because there is first amendment protection against libel accusations if the representation can be regarded as satire or parody. I think this is considered as “fair use” legally. Think political cartoons.