Sorry just got stuck and smiled over your first sentence "Fragments were way over engineered." and couldn't restrain myself of thinking about all Googlers that passed the whiteboard tests
True I don't think I have ever had an interview question that questioned my ability to keep simple. I think this is where experience comes in valuable, the ability to "keep things as simple as possible and no simpler".
I find the best approach to KISS is to write it as simple as possible under the assumption you'll have to throw it away. If it turns out you don't have to (after measuring), then all the better.
I host interviews for Google. A simpler design that solves a problem is definitely valued better than an unnecessarily-complicated one.