I think that the violence and the reader both evolved and matured with the series.
Early on in the series KA used a lot of cop outs, such as Tobias' line where he won't describe the violence because it will be "his own personal nightmare". There are a few violent scenes, specifically in #4 when Marco is attacked by a shark, but violence that pertains to actual Yeerk vs. Animorph battle is limited. It's actually a bit more like you would see in a kids tv series (Teen Titans comes to mind) where the bad guys either fall by the wayside or one of the two sides ends up running away.
As the series progressed, so did the amount of violence that KA was comfortable detailing. Let me emphasize that the level of violence probably didn't increase, just the amount that was given detail. I think probably books 1-6 were pretty blase on the violence count. #7, for me, is the first one that sticks out as having a really gruesome battle, when Rachel fought that battle on the tower in her Grizzly morph and lost an arm. Still, the grizzly has conveniently weak eyes, so not too much could be actually described in a visual sense.
The violence sort of hit its stride with the coming of the David trilogy. Rachel biting her tail off was definitely an intense moment. You also have David hitting Ax with a baseball bat so hard that part of his beak falls off, and the whole Jake-gushing-blood-out-of-his-neck thing.
Progress further into the series and you have Ax posing as a torturer, many many throats being ripped out, a few dozen (or hundred?) Taxxons exploding their gooey entrails and so on and so forth. In my mind this kind of culminates in what I think is Megamorphs 4, Back to Before, when Jake faces the dying Controller who keeps saying "I'm cold." and begging for a chance at life. Jake, knowing the Yeerk and the human will die, turns and walks away.
Flash forward even further and you have the massacre of tens of thousands of Yeerks in the final battle and, of course, Rachel's death and the deaths of the auxilaries. All in all it ends on a much bloodier note than it began.
Taking all of that into consideration, lets now look at the reader. The series started in 1996 and it has a reading level of 5th grade. That means that the goal audience was about 9-12 years of age (on a side note, I was actually in 5th grade when I started reading them and I was ten). The question then is, can a 9 year old handle the level of violence portrayed in, oh, say, book 32? Maybe not if they just picked it up out of the blue, or after the latest Babysitter's Club mystery. BUT, after the 9 year old has read the first few books, which introduce the reader to the violence slowly, he/she becomes desensitized to the fact that people die and it can be, and in the Ani's case is, bloody.
I also think that its important to mention that KA's message would not translate well if it were a non-violent book series. Also of note, AppleGrant are kind of notorious for violence and the generally weird. Remnants has a scene in which a character is split almost in half by a gigantic knife, Everworld has the main character's walk over a road of humans buried up to the neck in the fourth book, and GONE practically has a lynching scene. They aren't afraid to 'go there'.
So, that was the long answer. The short answer?
No, it was not too explicit. The violence was necessary to effectively convey the message that Applegate was trying to portray. Additionally she approached it in a way that allowed for the audience to become accustomed to the violence over time and, therefore, not traumatized by it. In fact, the violence allowed for enough 'realism' to make a science fiction children's series realistic and even believable, and kept the readers hooked and waiting for more.
/endnovel
