The only relevant question here: Was his original election legitimate? Did he get all the votes ascribed to him?
Not so. Did he assume dictatorial powers for himself after being elected?
Maybe, maybe not. This is not the only question though. He was elected to a presidency with very low power. 2004 Constitution amendments passed with 90% of MP votes made it so.
He was able to maneuver and abolish the 2004 constitution citing technicalities 6 years after it had been passed and started to rule by post-soviet constitution giving the President enormous power. A totally different position he was never elected to and that 90% of MPs had voted must not exist in Ukraine.
With his illegal power he fixed the parliamentary elections in 2012 and was ready to become a President for Life as all the other Putin’s buddies in the region are.