Is there a committee that monitors the decisions made by umpires, other than the match referee, and will their findings affect the umpire's reputation?
In international matches the ICC log and monitor the decisions made by umpires. According to Simon Taufel (a recently retired umpire from the Elite Panel):
ICC has taken the performance assessment of umpires very seriously as our rankings and continued contractual offerings depend on it. Every appeal is logged by the ICC Match Referee, every appeal is reviewed in the office and is assessed into three categories - correct, incorrect or inconclusive.
An umpire who is performing unacceptably poorly is therefore unlikely to remain on the Elite Panel, assuming there are better umpires waiting to be appointed.