Should I read Count Zero before Mona Lisa Overdrive?

In general it is "safe" to read any of Gibson's books in any order. Books within any of the trilogies tend to have enough distance from each other than "spoilers" are rare and don't detract significantly from one's ability to enjoy the prior books. The trilogies tend to be bound together by coherent themes and concepts. So you will better appreciate the trilogy if you read it in order, but your enjoyment won't be significantly harmed if you don't.

I've read both the Sprawl and Blue Ant trilogies multiple times, and found that my appreciation for each book increased on further readings. I know the plots well, but Gibson's genius is really in the texture and subtle detail.

I would almost suggest that if you believe that a book can be 'spoiled' by knowing its plot elements that you read a Gibson trilogy in reverse order to see how true this isn't of good literature. But then I have little patience for literature and film that relies entirely on a plot twist and is 'spoiled' if this twist is known ahead of time.