Powers of the local executive are not always as great as one imagines, between the state on one side and city council on the other, but I'm still reasonably sure running a city of 9 million people, even if you don't get to do much other than ribbon cuttings and elementary school assemblies, can take a bit of time.
Aside from the question of whether de Blasio is Good or Bad generally, I do not get thinking well, hey, this job is kinda boring so I'm gonna go run for president too. Especially as he is not exactly known for his 16 hour days.