I wonder (as other posters have said) if Margo hoped Claudia and her children would be leaving before the meal, or just didn't have a chance to rearrange her table. That was the only bit where I'd say Margo may have been rude.
As for the rest, as another poster said, Claudia initiated every rebuttal Margo gave. She wanted reassurance it was fine to bring her uninvited children (it wasn't), then wanted her children to have full meals, then wanted to know why Nemesis got a full meal when Claudia arrived first. Wow! Claudia has nerve, hasn't she?
Good for Margo for giving Claudia's dinner to the two children. Whose meal was she expected to hand over? Her own? She hadn't catered for extra guests as she hadn't invited any extra guests, and I don't see why she should have to do without her own meal to cover Claudia's guests. (I count them as Claudia's guests as she, not Margo, invited them.)
M2kbug, I think the pasta making hostess was at another party that Claudia crashed, and there were no Clue-by-Fours applied to Claudia's head at that party.
I'd hope that this might be a wake up call to Claudia that she can't treat people like this, but, since she's got away with it on at least one other occasion, I doubt it.