A bad player at the table can affect you, especially if it gets you angry or worked up enough to affect your focus or game.
More seriously, in the short term a bad player who goes bust by making bad calls takes high value cards out of the running for other players, which affects your chances of beating the dealer. In the long run, the hands should average out so your odds are about the same. Mind you, if the player is really that bad, ask him if he wants a friendly low stakes game afterwards.