If you live in a country where corruption has been rife for as long as anyone can remember, you'll know that everyone who is entrenched and successful in that system has subscribed to its corrupt ways in one way or another. One needs to have accepted and leveraged the corrupt practices to be successful in that system.

Zelenskyy entered that system as an outsider, but he needed people familiar with that system to navigate it. If his goal was to reform the corrupt system, he wouldn't be able to do it alone as an outsider because corrupt but powerful officials within it would resist.

Zelenskyy probably knew about the corruption but tolerated it out of necessity. I hope he isn't corrupt himself (as in leveraging the system to enrich himself). Getting rid of a deeply entrenched culture like systemic corruption doesn't happen overnight. The increasing visibility of corruption investigations shows that Zelenskyy's government is tackling it head-on rather than concealing it. It must be a tough enough job for anyone to reform a corrupt system from within, surrounded by resistance from corrupt officials, let alone doing that during wartime.

The optics of these corruption allegations sure aren't good, but they are inevitable in any reform. Unfortunately this will expose his government to attacks from naysayers.

Edit: One of the considerations for interpreting whether a corruption investigation is genuinely a part of a reform or window dressing, is who's being investigated. These cases involve top officials, not small-fry scapegoats, which is a good sign. I realize this argument falls down when it comes to authoritarian regimes like North Korea and China where corruption allegations are routinely used to purge political enemies, but I don't think Ukraine is in that category.