Haaland scored twice and also set up Tijjani Reijnders' goal as City's pre-Christmas stroll at the Etihad Stadium kept the pressure on Arsenal in a gripping title race.
But although City's fifth successive league victory took them to the top of the Premier League before Arsenal's visit to Everton later on Saturday, Guardiola is not convinced by his team's performances.
"I'm happy, I cannot deny that, we are fourth in the Champions League and in the semi-finals of the League Cup, but the way we played with the ball we have to improve," he said.
"Otherwise, it will not be enough to arrive in March and April to be contenders to win titles.
"I want to be honest. I said to the players, 'Merry Christmas everyone but it will not be enough if we don't improve'."
Guardiola has led City to six Premier League titles during his illustrious reign, amassing a wealth of knowledge about how to succeed in the gruelling English top-flight.
Concerned by City's lack of cutting edge at times against West Ham, Guardiola demanded they show a more ruthless touch when dominating games.
"In the first half we controlled the game but even with that we were not in the right places to create and we talked about that. We had to create more chances to be solid," he said.
"In the second half when they made a step up we continued to play not good. We did not do the right process to do what we have to do.
"If they had scored to make it 2-1, West Ham would have made it 2-2."
Reflecting on the positive aspects of the victory, Guardiola praised Haaland for a typically clinical brace, as well as the dynamic displays from Phil Foden and Nico O'Reilly.
The prolific Norwegian has now scored 38 goals for club and country this season.
"There were many good things. Offensively, Erling. I always have to thank Erling for the goals but today thanks to him and Phil and Nico they helped us to do this type of game," he said.
"Defensively we are improving. We are visualising it with the ball."