It is from what I've read and interpreted, all Luffy does is just give the locals a fighting chance to save their country. Most of the time, the problem isn't that a king exists but that some foreign entity like a pirate group, marines, world government is trying to take over an already vulnerable country.

The time when it actually is a bad king the kingdom becomes an electoral democracy run by the people headed by the those who fought for the freedom of the country. Oda makes sure that the people themselves have agency in liberating themselves and it isn't some one sided savior complex. Even in arcs where it seems like the straw hats are doing everything Oda still manages to tie it to the people of the island.