Cellulite is when your skin is not strong enough to hold the fat deposits underneath them, so the structures collapse on themselves. I don't think it's unhealthy on its own but it's definitely not a sign of good health.
Obviously it's not attractive but it's not really that huge of a deal.
One thing to keep in mind about cellulite is that it's more possible to get rid of it than most people think. You gotta be both in shape and also eat well (not too much, and also only eat healthy foods) but it's possible to do it.
I think it's sorta like how it's possible for any guy to get chiseled but most guys stop their efforts short of actually getting there.
Like you said, over 90% of women have it. It wouldn't bother me I feel women make too big of a deal about stretch marks and cellulite, neither would turn me off and neither are just confined to girls, there are guys that can have one or the other or even both.