This speech is rightfully regarded as one of the most important in the series, and I do think it's important that Doflamingo is the one who says it. Out of everyone in the manga he's had the most diverse life experience. He’s seen how malleable concepts like justice are and knows how easy it is to manipulate a narrative. History is, after all, written by the winners.
What's important in the larger context of the chapter, though, is we see the start of that narrative in action. Two pages ago Akainu was extrajudicially killing a frightened soldier while hinting at a grander plan to take out Whitebeard through deceit and trickery. With the power of retrospect we know that the marines planned on cutting the livestream, using the lack of transparency in order to trick the general populous. This of course doesn't work, but the intent was there. The World Government was perfectly content with using ignoble means to achieve their so-called noble goal, but were willing to lie about it in order to maintain face.
Secondly, I think it's quite important that in this last panel of Doffy's speech...there are no named characters whatsoever. The winners decide what Justice is, but their definition affects ordinary people in life-changing ways.
Even a character like Whitebeard, who tries to protect his sons and offers chances to flee to those who no longer have the stomach to fight, isn't exempt. Earlier this chapter he was ordering his sons to use the body of one of their siblings as a glorified bridge. While we understand and sympathize with his reasoning, there's no getting around the fact that he started a war for the sake of one person. He has sixteen divisions and forty-three allied pirate crews rallied to his cause, and thousands die because of his decision. What makes Ace's life more valuable than any of these nobodies that history will inevitably forget?
It's a difficult thing to grapple with, and there aren't any easy answers, which is exactly why it's one of the most profound and important speeches in all of One Piece.
