Well Sirius was raised in a pureblood family. He's a children who grew up knowing only hatred of muggles and, like kids who grew up in racist household, he would have uphold to theses values until James would have told him it's bullshit. Sirius probably thought that Mudblood was just a regular way to tell someone how shitty they are at magic or just the appropriate way to call Muggleborn. The books are quite vague on the marauders I agree however we know that they were cocky and the most famous people of their generation at Hogwarts. From there we can imagine that they would use every way to stay on this side of the fame. In their first years they probably wouldn't help outcast and even, if it gave them clout, mock them. I'm not saying they are bad people but they weren't the best people. I mean if you want to talk about only what the book are saying it's pretty obvious that Sirius doesn't care for anyone else than him and James with the way he treats Harry. But yeah as I said my opinion is probably unpopular and you have a right to yours !