In medieval culture, an event like a royal christening is not a private party; it’s the public social event of the year. To not invite any person of rank to such an event is a deadly insult.
Maleficent is certainly someone you wouldn’t want at a party, but she’s also someone powerful enough that only a fool would ever dare treat her with such blatant disrespect. The only way the King and Queen could possibly have gotten away with not inviting Maleficent was to not invite any of the fairies at all; inviting the other fairies and excluding her is explicitly taking sides in the conflict between the fairy factions.
Which means they made themselves her sworn enemies, and she responded by treating them as such from then on. If you actually get into analyzing the social dynamics of the scene, it’s very clear that Maleficent was willing to show mercy at first by giving the King and Queen a chance to apologize for their disrespect to her. She doesn’t curse Aurora until after she gives them that chance and they throw it back in her face with further disrespect.
And yeah, if the King and Queen had done the properly respectful thing and invited her, Maleficent would have given Aurora a scary awesome present. Moreover so would the other fairies, because at that point both sides would be using it as an opportunity to show off and one-up each other. What they gave her before Maleficent showed up was basically just trivial party favors by fairy standards.