Ukraine could've done much better if they'd mobilized and fortified their borders before the invasion. But Zelensky refused to do so (despite warnings from the U.S.) because war with Russia was hugely impopular with voters in Ukraine before the actual attack. They'd much rather stick their heads in the sand.

So a lot of their territorial loss can be attributed to his inaction. If they'd laid several million landmines in the Donbass the Russian attack would've been stumped and the occupied Crimea would've been easy prey.