Maternal personality probably does have a role - but without decent studies we can't say in which direction.
It could be, for example, the very stressed, uptight mothers are the ones who manage by sheer force of will to get through the very stressed, uptight days of early breastfeeding, while the laid back, relaxed mothers think, 'Aw, sod this for a game of soldiers, he's going on the bottle!' (Sorry, just seen Kveta has made the same point!)
umf - I have a background in research and studies I have been indirectly involved in have used the SDQ tool and while parental perception and assessment is indeed crucial to it, this does not invalidate it. It has been validated and found to be a robust way of assessing behaviour and because of the way the questions are asked, and what the questions actually are, you get a meaningful picture. Parents are the only people who see their kids over time, and the pre-school workers you say would give a better assessment would still have to ask parents questions....and of course all parents were asked the same questions in this study so you would hope this would bring consistency.
Yes, there are drawbacks in parental perception questionnaires, but this one is a good one which overcomes at least some of them.