Considering Ronald McDonald was a character primarily aimed at young children, I don't think they were mentally capable of having personal responsibility at that age.

As for the parents who were pestered to buy happy meals by their children, there's like 50 ways to answer this question. I personally think that in a mentally healthy adult, personal responsibility is a factor, but it's not the only one and is balanced by social conditioning, genetic predispositions, mood in the moment, and a ton more factors.

The children who for one reason or another were brought up eating fast food are conditioned both socially and biologically to eat fast food, and breaking out of that addiction (as with any other addiction) can be very difficult, and is more complex than doing the equivalent of saying "git gud scrub".

Wow, that turned into a wall of text, sorry.

Tl;dr: it's way more complex than just "personal responsibility".