Here's an opinion on Clark from a war correspondent that was in Serbia during the war. He doesn't speak too kindly of the general.

Frisk interview

As a caveat, the reason Clark was fired by Cohen was because he didn't approve of the Pentagon's insistance of an aerial campaign. He felt the Serbs were too entrenched to simply win by bombing. So he bypassed Shelton and Cohen and got authorization to use more ground forces.

It proved to be the right tactical manuever, but ultimately cost him his job.