she lives in Germany! She’s lived there since she was 13! She hasn’t said anything at all about the war! THE STRAW HATS ARE TERRORISTS! NICO ROBIN WORKED WITH THE 
DURING THE TIMESKIP! YOU WOULD 100% CONSIDER THAT TO BE TERRORISM! WHAT THE FUCK ARE YOU ON ABOUT 
Yeah but they’re generally monarchies that support the people and have the people’s support. It’s a weird dynamic but it feels intentional to avoid the whole “and now Luffy and the straw hats stay for a month to guarantee a free and fair election” thing that would slow the already glacially slow story down
It does kinda undermine the world building that’s supposed to be oppressed under the world government when you line up all the monarchs under them and half of them are our buddies from the last arcs
It doesn’t undermine it at all. It’s hard to answer this question without spoiling the entire series but I can just say that there is no choice of government within the OP world that isn’t a monarchy since the world is controlled by an empire which calls itself the “world government.” That’s to say, It’s not a coincidence that Luffy and co. keep stumbling into monarchies and leave them as monarchies. There is a world police force called the marines that enforces this social order and each “kingdom” (which are really just colonies) has to pay a tribute to the world government to simply exist.
The WG doesn’t recognize governments that are not monarchies or at the very least doesn’t protect them. The majority of the world is dependent on the WG’s military force of marines and intelligence agencies for protection from pirates. Those pirates were created from the conditions of enforced poverty and servitude that the WG set (the lumpenproletariat) while the working class inhabitants of the world have their labor value taken from them by the WG in the form of tributes. In exchange, the monarchs of these countries live lavish lives (nowhere near the level of the WG) and have slight autonomy in how their affairs are handled.
There exists a revolutionary army that wants to destroy the WG which would upend the entire social order that has been constant for centuries. Luffy is not part of that army since he’s a pirate, therefore he doesn’t do any organizing in these islands since that’s not his goal/role.
EDIT: In an interesting sense, pirates must exist in order for the marines to justify their control. It’s only when these pirates grow too powerful and amass territory that is independent of government control that they are considered threats which then justifies the hyper-militarization of the marines which continues the cycle until it reaches a breaking point and a revolution occurs like we have in the current narrative of OP.
I mean the places that don’t line themselves under the world government get designated as islands where nobles get to hunt slaves for fun, so it’s not like they have much of a choice.
spoiler
or worse they just get hit with a space laser and removed from existence
Is there any chance this is like a Bolivarian-thing? Where the regional bourgeoisie team up with the proles/peasants, etc to expel the colonialists/imperialists?
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.
Also they are not monarchies in a sense because they are all under the control of the world government. They are essentially puppet governments that allow the WG to extract and hoard wealth while also giving the illusion of political autonomy/diversity.