Why Force Z needed fighters more than another cruiser

Would one or two Dido-class cruisers have saved Prince of Wales and Repulse?

They would help, but fighter cover from almost any carrier would address the fundamental problem far more effectively.

One or two Dido-class cruisers would add useful anti-aircraft fire around Prince of Wales and Repulse. The decisive deficiency was the bombers' freedom to form and begin their attacks without disruption before entering gun range. Even a small carrier could put fighters far enough out to break up those formations and spoil their runs. Early interception offered much greater leverage than another layer of local anti-aircraft fire.