I think Kup summed it up pretty well, Kawamori made changes to Starscream inkepping with the sort of designs he creates, Sleek, semi-realistic. Generaly speaking the stlye of G1 is beefy and block like so having someone who designs lithe mecha with long flowin fins and the like is obviously going to be moving away from G1. I happen to like the original MP starscream because he is a Kawamori interpretation for Starscream, but I do agree that he is divergent from Prime who is basicaly a recreation of G1 cartoon prime.

I think his work in Macross shows that he is maybe running out of ideas when it comes to developing Valkyries. The VF-25 looks like .Valkyrie Remix' - Mostly a VF-0S mixed with the SV-51 and some YF-19 as well. It is possiby his most refined Valk to date tho and a beautiful design.
I think a lot of artists when they get older get stuck in there ways, and tend not to try anything as revolutionary as they did in thier youth. so all Kawamoris new Valkyries seem quite similar, also working in 3D has forced him to abandon the cartoony changes in proportion common in his older valkyrie designs. (the VF1's wings and nose cone, and the YF 19's legs are a good example of significant changes in proportion)