>>91290190Jackie has every reason to feel hurt and betrayed in her situation. She doesn't know Star like Marco does, they are (were) friends but they're not close and almost all of their interactions revolved around Star trying to set her up with Marco. And now that they're together, Star confessed to him and tore herself out of his life.
Jackie genuinely likes Marco and has always trusted him despite his fixation with Star, even with them living together, because she knows Marco's had a crush on her for a long time and Star's been working to set them up almost since she arrived on Earth.
But now there's doubt in her mind, even if it's not Marco's fault, even if Star confessing to him is an implicit confirmation that they had never done anything while they lived together.
They were supposed to be enjoying their first summer together as a couple, and now once again he's more worried about Star than her. He had to check on her after their first kiss, he (from Jackie's perspective) invited her to a concert with them, and now he's going to just ditch her for who knows how long to find his "friend" who chose to leave him.
"Mad" might not be the right word, but she has every reason to be upset.
