No need for Pep Guardiola to sign new players to assist Haaland, Kevin De Bryune is there to assist him but due to the injury that both players suffered towards the ending part of last season made them not to be able to Perform as they normally do, instead of signing more players to the team Pep Guardiola should consider using some of the players in substitute to balance up the challenge that Haaland is getting and Guardiola should not be overdependent on Haaland alone, he should try and build up other players in the squad so that in a case of injury he can have a reliable replacement for Erling Haaland. Haaland still shine even when playing against a strong opponent so saying that he only performs when they meet a mediocre team is not true. Remember he scored a hat trick in their derby match against Manchester United last season so except you will classify Manchester United as a mediocre team then I can agree to what you said.
One thing you need to know is that Manchester City, even though they are one of the strongest contender of the Champions League but they are mostly unlucky in winning the Champions League so even if they sign new players, don't think it will make any difference.