I think Clardy is grumpy about Jeff partly because he is just something of a jerk. It seems that he hates anyone who seems like they're going to think or do anything like that.
But I think it's also partly because of the plot that Clardy is part of. I think that if he is a jerk to everyone, they'll be a bit afraid of him and that will make them not be suspicious of him.
So I think it's partly a personality thing but also partly due to the fact that Clardy is being a traitor to the Union, if not to the evil aliens of Zuberka.