It isn't necessarily wrong, although a healthy council will usually aim to distribute responsibilities among members. But if there is no good alternative, and the council chairman carries out the role well, it may be acceptable for them to also chair the finance committee. However, if the chairman is attempting to have undue influence in the council, that should be resisted. The council really should be a shared enterprise as well as being responsive to citizens. Sorry if that doesn't give an explicit "right or wrong" answer!