I don't think its necessarily the fact that they're involved with the McMahons that gets them over, but rather, the fact that the writers actually give the wrestler a storyline and a push that they FOCUS on.
Think about it. Right now, the main story on Raw is with Orton, correct? How much focus is it getting? Constant. Its not just Orton coming out and squashing someone for 40 seconds and the announcers saying "watch out for this guy". Its giving the fans something to play off - something to pay attention to and follow the progression.
Now, what are some of the other storylines that helped get people over recently? There was Jericho/Michaels (didn't include the McMahons) and Edge/Taker (didn't include the McMahons). These helped Jericho and Edge greatly and its because they were actually building a feud that had a purpose to it other than "these two are in a feud, so, pay attention".
Now look at the unsuccessful feuds that have happened. Kane and Mysterio went all over the place, had no clear direction, had the same match over and over again (Mysterio dominates Kane by kicking him in the shin 1000x and then does a splash and beats him), and there were weeks where we wouldn't get ANYTHING for the feud. Literally, nothing at all. Then there were a few weeks where the only thing that had to do with the feud was Kane + Mark Henry versus Mysterio + Partner, again having the exact same match repetitively. R-Truth and Kendrick are in a feud right now, I guess, and if they didn't wrestle each other or no mention of it occurred tomorrow night on Smackdown, would anybody notice? Nope.
The WWE has a tendency to want to push one person at a time, throw all their focus on that, and then the writers scramble at the last second to throw a couple of other people into mini-feuds that consist essentially of Person A beating Person B one week, the opposite happening the next week, and then a third rubber match. If the feuds that have no focus aren't of that formula, then its simply "heel attacks face, face has a match with the heel but loses (or wins and is attacked), they have another rematch and the face either loses or wins and is attacked..." So when they actually have a storyline that they want to use where they really want someone to get over in a big way, they actually take more than 20 seconds to think about it and they pay more attention to it, and that focus leads to a more interesting product, which leads to the audience being more entertained, which leads to them associating entertainment with that performer, thus resulting in them booing or cheering him according to his status.
The McMahon connection isn't simply "McMahon + Superstar = Superstar gets over", its "McMahon + Superstar ----> Creative pays attention and puts actual work into it = Superstar gets over".
Unless of course, its such a god awful idea or so poorly executed that it couldn't possibly work for anybody. Cough*illegitimate child*cough.