I think K.A hit the right balance with Everworld in terms of swearing and innuendos, and if she had done the same thing with Animorphs if it had been marketed for an older audience, then I think it would've been fine.
As for a high amount of swearing though, I think it would've been ridiculous to have that in Animorphs, mainly because you have to remember that these are 12-13 year old kids. They are going through hell, with enslavement, morphing changes,... death, but they are still kids. They're not gonna go "f*&k you" again and again with every battle when they're still too young to pick up those types of habits. I could see it happening in the final arc maybe, since they are 16 at that point, but before that it would've been OOC for them. You don't need swearing to make the series more mature or "darker". As Nohenson said above, the boundaries were already being pushed for a middle grade series.
Also, sex scenes? Really? They're 12 year old kids. I could see rape/forced breeding coming into play, but again, I didn't feel the series had to go that far to be mature enough.