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It's not standard procedure, since Thahn, Haley's second in the Resistance, was clearly not a Lawful Stupid paladin, and he was also one of the paladins regularly sent as a messenger. Miko is the only example we see like this, and it's made clear that Miko is an isolated case. Hinjo doesn't say they only use assholes for messengers, he's saying Miko, specifically and uniquely, is such an asshole that they like to get rid of her for months at a time.
Remember the Party's first meeting with her: Roy and Durkon, on learning Miko's a paladin, don't react with "Oh, she's a paladin, she's lawful stupid." They say, "Oh, she's a paladin, she's clearly on the side of good," and it's only after prolonged exposure to her that they see her as an asshole, so it's clearly not a case where the world thinks Paladins are Lawful Stupid because they all only send out the worst of them to see other civilizations.
Bottom line, it's a single case in only one of what's probably many orders of paladins, and you're saying that means all paladins everywhere act like that.
It factors in some way - Hinjo devoutly believed that all his paladins were too Lawful Stupid to be of use in anything but a stand-up fight, and most of Azure City didn't even know they existed.
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