Do I belive him (Zelensky)? Well, we do have that single peer-reviewed article. Let me quote from that:
"We find no evidence of systematic bias in Ukrainian reports of Ukrainian military deaths."
And:
"We compute the ratio of casualties to deaths for Russia and Ukraine, finding values of 2.9:1 and 4.9:1, respectively."
That would give a total of 150k Ukrainan casualties, which is a bit lower than but farily close to estimates from other sources.
Russia's and Ukraine's incentives to lie are different. Russia primarily targets their domestic audience, who already knows that the regime is lying through their teeth, so they have to lie even more to make people think that the war is going well for Russia.
Ukraine's primary concern is western support - domestic support will likely be relatively high anyway. Blatant lies erodes trust in the Ukrainian government, which will impact aid. If anything, Ukraine is more incentivized to overstate their own losses, and, by extension, their aid needs. Both Ukrainians and "westerners" already know that the war isn't going great for Ukraine. If the real KIA figures were 50k or 75k, stating that wouldn't erode either foreign or domestic support.
Ukraine simply has no reason to lie, every reason not to, and independent, peer reviewed science claim that they don't lie. Do I believe him? Not blindly, but I see no reason not to in this case.