It is really impossible to tell as we only really see him and hear about him after he has been put under the Imperius curse, so it is unclear what his actual motives, ideals, performance under pressure, or political leanings would be.
Yeah, unlike the movies, where he was willingly allied with no Imperius Curse on him and was then killed ruthlessly by Voldemort just for interrupting him.
@Harrypotter394 no, Thicknesse was not killed by Voldemort for interrupting his concentration on the destruction of Ravenclaw's diadem. It is unknown how he died in the book or if he even died at all. And in the book he WAS under the Imperius Curse cast by Death Eater Corban Yaxley, read here.