City moved top of the table ahead of Arsenal's visit to Everton later in the day thanks to two goals from Erling Haaland and one from Tijjani Reijnders.
It's now seven wins in a row in all competitions for Guardiola's side, but it wasn't enough to impress the City boss.
"The way we played with the ball we have to improve," said Guardiola.
"Otherwise, it will not be enough to arrive in March and April in contention to win titles.
"[West Ham] is a good team. But I want to be honest with myself. I said to the players: 'Merry Christman for everyone, but it will not be enough if we don't improve'."
City were in control against West Ham from the fifth minute when Haaland scored his first of the afternoon.
Reijnders made it 2-0 before half-time, but West Ham regrouped in the second half and had early opportunities to get a goal back through Freddie Potts and Jarrod Bowen.
Haaland ended any chance of a comeback with his second goal on 69 minutes.
And although Guardiola said he had to "thank" the Norwegian for his goals, other players including captain Bernardo Silva, were given a dressing down in the post-match news conference.
"Bernie is my weakness. He's top, but today Bernie had to play better. Today I'm not happy with Bernie," said Guardiola.
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"He's my weakness, my favourite one, but today I'm not happy with him.
"He knows exactly what we need to play better but he has a special sense to compete and play and incredible attributes in the bad moments to step up.
"He's reliable, can play in five positions and that is extraordinary but today he didn't play good."
