Severus did not make the comment to Lily, but to James when taunted about Lily's coming to his aid.
In some way, I think that makes it worse. While we're at it, let's break down this "things said in stress/anger and whether you truly mean them."
Mudblood wasn't going to hurt James. Actually, the only reason why I could think Snape said this to James was because he was trying to ~remind~ James that Lily wasn't up to his (Snape's or James') status and therefore the quality of her help was less than the quality of Snape's overweening pride and arrogance self-determination. He was reminding James that Lily has so little social status that her help might as well have been non-existent. She shouldn't even be considered in a James/Snape fight (because they're clearly better). In addition, saying it to James further dismisses Lily as a viable actor.
Now, when you get in really big fights with people, sometimes you say things you know will hurt them, but you don't actually mean them. It's basically picking at your adversary's insecurities. It does happen. It just didn't happen here.
Snape made a comment about status. Sorry, Snapefen, when you make status comments that reflect how you feel about your status, you always mean them in some way. Actually status comments are the ones that are usually more truthful when said in anger, since you're not as worried about seeming arrogant or offending people. He meant what he said. He might have thought Lily was the best of the "mudbloods", but that doesn't do him any credit.
Also, if Snape wanted to make himself feel better by putting someone down because he was stressed out, James was right there in front of him. He didn't have to say anything about Lily to take a shot at James. And I think he did in someway go for James' insecurity: Lily's importance to James. James was trying to make the fight about him and Lily, and Snape wanted the fight to be about him and James again. So basically he decided trashing Lily would make a point to James. He decided to sell out his best friend to take some cheap shot or make a point to his enemy. I don't care if he was in a red rage or he meant it or not, selling out your best friend should never, ever, be an option. Seriously, friendship dead. Once my friend starts thinking this, our friendship is over.
So yes he said it in anger, yes he meant it, and no, it was no excusable because he said to James.
I meant to add: So basically Snape was like, "Fuck Lily, Imma get mine." At this point Lily probably thought, if Snape could deride her existence as unimportant for a petty schoolyard fight, what if those little douches he hung out with made him make a choice on bigger scale. So she told him, her and them, and not choosing in this case was choosing them. /rant
Why did Snep call Lily a 'filthy little' *anything*? Even if he hadn't used the M-word, if he had said even just 'filthy little girl', that'd probably have offended her just enough to break the friendship, wouldn't it?