First of all, Kuzan's hands being in his pockets means absolutely nothing. Kuzan doesn't have a power that requires him to physically exert himself. He froze miles upon miles of water for five whole days just by dipping his fingers inside.
Kuzan warned him, but Doflamingo took his attention off of Kuzan and put it back on Smoker, as Kuzan froze him while he was delivering another attack to Smoker. Nothing indicates that Doflamingo took so much time to break that it'd be a window for Kuzan to attack him, not to mention that nothing indicates that Doflamingo couldn't have avoided it if he was paying attention to Kuzan instead of Smoker.
Power-scaling. Kizaru could stand up to Whitebeard and keep him from moving forward, but you think he's less tough that Luffy? Kizaru took a kick from Marco, someone who could push Akainu, and didn't receive a scratch, but Lufy was injured by someone who can't beat Admirals(Doflamingo).
Luffy is above all the Admirals in brute strength? So you think Luffy is above Whitebeard too in brute strength?