Well, the "Buffy" who was in the movie version was blonde. I think the "joke" is that the savior of the world (The Slayer) is a ditzy, blonde Valley Girl.
Another reason he might have gone for the "blonde" would be that Sarah Michelle Geller is a much better actress than Charisma Carpenter.
Although, I will say that Charisma Carpenter did a lot with that character and was very entertaining. I sometimes think she really got the best one-liners.
You’re right. SMG is really good. She’d been acting since she was a teenager.
And CC did get some funny lines. “I hate to interrupt your downward mobility...” (ep one, season one).