Does anyone know why in the last scene when Zack is being asked for his autograph, Amy puts this sad/serious face?
Is it because he is gonna be crazy again about being famous or because she just kissed him so he becomes more famous?
Yes, it's because she's worried Zack caught the fame bug again. From the Wikipedia episode synopsis:
Everyone (including Zach and Amy) begin chanting "Zach! Zach! Zach!" After this, the crowd members ask him to sign autographs and to take pictures with them. As Zach does this, he starts singing "Zach Stone Is Gonna Be Famous" to himself. The final shot of the show shows a close up on Amy's face as she watches this unfold. The smile she has had since the beginning of the previous scene slowly begins to fade towards a disappointed, saddened expression. She knows that, what with all of this fame and recognition, that Zach is probably going to go back to his old ways within time.
So this was either intended as a bitter-sweet ending, or an attempt to maintain some of the main premise for next season - but the show was unfortunately cancelled, so we'll never know for sure which.