In the interests of accuracy, I went over PMMM's scripts (
here) and checked everything that Kyubey said throughout the original series. Based upon my readings, I would dispute that he actually said this. He has a number of lines that mention despair or curses, most of which seem like they're referring to witching out, such as this one:
The reference to "her eventual despair" seems fairly obviously to be describing the power of her witch. And we know that girls aren't "destined" to witch out because they can die before that happens, resulting in no witch ever appearing- there's no influence of fate there. But also relevantly to the relationship between potential and a girl's circumstances was this:
A queen or Messiah would be expected to have higher potential than a normal person. Being "the focal point for so many destinies" is implied to have a relation to "the weight of the karmic destiny she bears", which in turn determines potential- that is to say, the more people whose destinies are tied up with a girl's, then the stronger her potential as a magical girl. You could invoke Kyubey-as-tricksy-devil to weasel word around this relationship but it seems direct enough that you'd have to make a significant effort.
As far as I can tell, every time that Kyubey draws a relationship between the potential/power of a magical girl or her hope and despair, he is (or easily could be understood as) talking about witches. The ones who talk about that relationship in pre-witch terms are Sayaka and Kyouko, neither of whom even knows that magical girls turn into witches at that point. So any understanding that a girl's unhappiness during her lifetime is tied to her potential, or that she's destined to experience or inflict unhappiness every time she tries to be happy, is more likely than not to be philosophical bullshit that they've conjured up to justify their own miserable circumstances and decisions, backed by misunderstanding what Kyubey says since he hasn't given them complete information on the magical girl life cycle.
There's probably a PMMM general thread somewhere or other that we can take this discussion if necessary, as I think it might be growing outside the original question of the reasoning behind a certain GM decision.