Most politicians do. Zelensky does have close ties with Ihor Kolomoyskyi, who, yeah, is probably a mobster. And it's not like Z has a strong resume of fighting crime. He was a TV star on Kolomoyski's TV station playing the President. Like Trump, his resume consisted of pretending to be someone important.
But, the consensus view of the intelligence community and the State department was that Z was someone that was a move in the right direction for fighting corruption. And Lev/Igor had approached Kolomoyski seeking assistance in digging up dirt on Biden. Kolomoyski, unlike Firtash, told them to fuck off. So it's not like the anti-Biden coalition of morons was particularly concerned about Kolomoyski's checkered past.
It might be a reasonable defense of Trump's actions if Trump had done anything, literally anything, to indicate he was worried about corruption and Zelensky. There were no inquiries to State or the CIA. He didn't announce concerns to Congressional committees. It wasn't mentioned to the OBM, so it wasn't noted in the Congressional notifications on the delay of funds. He didn't even mention the concern to Z on either of the phone calls, except outside of the very specific concerns about Burisma and Crowdstrike, which there are no indications Z had connections to.