You are generalising that blondes appear to "fizzle out" mentally rather more quickly than their friends with other hair colour. The truth is we most likely do not ... we are perhaps just smarter at spotting whether a subject is actually worth pursuing
Which brings me onto another point. We have all heard the expression "blondes have more fun". This is absolutely the case!
OK, to be serious for a moment...
I'm not sure I'd believe any generalizations about people based on hair colour. There has been some recent news of some studies (the content of which I forget, alas - although if you really care I'll go try and dig it up) which found a slight linkage in something, but until I see actual data I'm going to classify all this as 'probably an old wives' tale' - and mostly likely they will be, is my guess.
Until real data is in hand, though, here's a point to consider the other way: you seem to be trying to say that blondes are more lively, and brunettes more dull? What about Italian/Spanish/Latino women - they hardly have a reputation as dull! And very few of them
I cannot for the life of me explain why blondes have been singled out though ... clearly this needs further investigation
Well, here's a theory
(emphasis to indicate how totally hand-wavy / out of thin air this is).
There is a perception that brains and beauty are somewhat exclusionary - and there may be some vague basis for this perception
(i.e. not the underlying reality), in that if someone is gorgeous, they just don't have to work as hard (i.e. use their brain, even if they have a good one) to get places in life. So even if there is no actual negative correlation between beauty and brains (I would guess there is not), there might seem
Now, add to that the taste of many guys to prefer blondes (why, I'm not quite sure, but paler skins/etc seem to be an attractor in many cultures around the world I can think of, as disparate as Japan and Easter Island, so there may be some underlying basis for that), and you have the outlines of a possible rationale: blondes are more beautiful, and beauty is negatively correlated with brains, ergo...
I would appreciate some sensible debate
Oh, we do do that too...
the crater I have already dug myself on the first day!
What crater? (No, that's not a joke... I honestly don't know.)