The former heavyweight boxing champion made the comments to Swiss news outlet 20 Minuten.
Klitschko was asked if he was surprised to see Zelensky's popularity declining across the country.
"No. People see who is effective and who is not, and there were and are many expectations. Zelensky is paying for the mistakes he made," he responded.
The Ukrainian President failed to heed warnings that his armed forces were ill-prepared and "denied until the end that it would happen," said Klitschko.
The Mayor questioned "why it was possible that the Russians were able to reach Kyiv so quickly" and chastised Zelensky for providing too much information "that didn't match reality."
Although he cautioned that Zelensky "will pay" for his mistakes, Klitschko urged Ukrainians that "we must support him until the end of the war," while ruling out any prospects of him replacing Zelensky as president, at least until the end of the conflict.
Klitschko's remarks arrive amidst a wave of disillusionment about the way the war is heading for Ukraine.
As we highlighted yesterday, Sascha Lehnartz, chief correspondent of German newspaper Die Welt, said the Ukrainian "counteroffensive seems to have failed" and that there was a sense Kyiv had "already lost" the war.