We generally have a policy that questions specifically about the SFF-nal elements of works, regardless if the entire work is on topic, then the question is on topic. This is a long-standing policy that's shaped many of discussions:
If the question is specifically about an sf-nal element, even if it's only a minor part of the work, it's on-topic.
So, we have to evaluate whether or not the question, Did The Lucky Charms Leprechaun always exist in the same universe as other General Mills characters?, which is about a magical Leprechaun (who coexisted with the magical Waldo the Wizard) and his connection to other characters is on-topic.
Fundamentally, the question is asking about whether or not a fantasy universe includes other characters.
To me, this seems to be specifically about a SFF-nal element of a universe (which has had varying levels of stories told throughout the years, via commercials, booklets, and packaging), that it should qualify under our rule.
The caveat being that this specific question seems to be on topic, but that's not necessarily the answer for the broader case of "marketing characters" or commercials, and it seems hasty to try and make a general use case.