Perugino's initial fame brought him considerable wealth and prestige, if not (i)enduring glory: some years after having been lauded as the most famous artist in Italy, his reputation having suffered a decline. Perugino was (ii) derided by the acerbic Michelangelo as an artistic (iii) bumpkin.

intial fame brought him wealth and prestige but it did not bring something, something related to glory. Reputation declined after some years, therefore, something that he did not have was a long lasting glory. Then, acerbic Michelangelo comments on perugino, since he is acerbic, his comments would be harsh, and hasrh comment will be -ve, therefore, derided and bumpkin.

Why not ephemeral but enduring?
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Because ephemeral means transient or for a momentum, for a period.

Here the sense of the sense was that in the initial period the fame was huge and this could be also enduring if was not for the post talent of Michelangelo that disrupted the Perugino's fame.
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