Jimmy was happy about how he had impressed the appeals panel, and Kim was happy for him initially, but as he went on it became apparent that he had been faking any apparent emotion regarding his brother's death. This bothered Kim, because throughout the season she's clearly showed concern for the fact that he doesn't seem to be sad or to have mourned that event at all.
From a script for the relevant scene:
Kim THROWS her arms around Jimmy! Smiling, laughing,
crying... so moved by Jimmy’s emotional performance.
KIM: I knew you could do it! I knew you had it in you...
JIMMY: (laughing) That felt... so good.
KIM: They have to reinstate you. They just have to!
JIMMY: (no kidding) Uh, yeah! God, didja see those suckers? I had them! And that one asshole at the end was actually crying? He had tears. Jesus...
Jimmy’s incredulous at what he’s pulled off. Kim pulls back a little -- what did he just say?
JIMMY: I mean, I started reading the letter, but I could just tell, I could see it on their faces it wasn’t gonna be enough, so I just kinda went off on this... flow. This energy going through me, like it was improv, or jazz!
Kim doesn’t interrupt him, but just listens -- stunned that he was faking. He fooled the board, and he fooled her, too.
JIMMY: Then I just -- pop -- sank the hook in. I had ‘em, like a pied piper leading the rats right into the river. Y’know, I could see it, I could see the Matrix. Like I was invincible. I mean, God... I can dodge bullets, baby.