Previously, it was difficult to be sure just what was going on with Miko Ino. An actual answer on whether she was depressed or not was subverted so many times; particularly if it was Shirogane doing the questioning, it would turn out Miko Ino was merely being happy-go-lucky with one of her strange hobbies, and then some ominous foreshadowing at the end would cast doubt on how to interpet what had just happened. But finally, in Battle 190: The Senior Classmate and the Junior Classmate, Part 3, and Miko Ino Can't Love, Part 5, has given a definitive answer that Miko Ino has indeed been distraught over Ishigami dating Tsubame. At the very end, when Shirogane just gets straight to the point (finally) and asks "Do you like Ishigami?", Ino replies with
I... hate him. He isn't serious. He flouts rules. He's stupid. He's not my type at all. But when... I heard he went on a date with Tsubame... I felt a sharp sting in the back of my nose... And my heart hurt. Shirogane... How do I handle this?
Ino is crying in the panel, and this conversation definitively and explicitly proves that Ino is indeed distraught over Ishigami dating someone else, and that is why she has been acting so odd. Notably, it does not explicitly confirm that Miko Ino is in love with Ishigami, and there could possibly be more subversion in the future (I don't expect there to be, but it's possible), but this is also not what this question is asking. This quote definitively proves Ino is deeply distraught, for whatever reason, because of Ishigami dating Tsubame. This answers the question, but I'm maintaining the rest of my answer because it offers additional details I find of interest.
Rest of the Answer
Due to my own lack of knowledge, I can't speak to the cultural differences in play here (though I do think they're relevant), but there is textual evidence to suggest that Miko Ino is distraught over Ishigami romantically pursuing another girl, which is likely making Ino behave strangely.
Though Miko Ino doesn't acknowledge her feelings to herself, it is implied by other characters and by how she secretly assists Ishigami that she probably has romantic feelings for him. For instance, Ino seems to greatly enjoy Ishigami waiting hand-and-foot on her while her arm is broken.
In Battle 158: Miko Ino Can't Love Part 3, there are many implicit suggestions. While talking to her friends, Miko Ino says,
Hey, I've been wondering... How can I explain this...? I've been trying to figure out... How come... Ishigami came over... afterwards, and...
From the text and from the strange look in her eyes, it seems Miko is confused about her own thoughts and feelings for Ishigami. Onondera's response is
"Well... Now's your chance... to make Ishigami your slave. He needs an outlet for his guilt anyway... so it's a win-win situation for both of you."
To which Miko Ino gives an affirmative response. On the other hand, Ino's friend Kobachi Osaragi sees trouble coming, leaving us with this ominous statement at the end of the chapter:
"Uh-oh. This is a bad strategy. Really bad. This is really bad. Miko is... going to realize it soon."
And this is said against the backdrop of a picture of Ino blushing while holding Ishigami's hand as he helps her up the stairs.
There is some textual evidence of Ino being a bit distraught. In Battle 154: Kaguya Shinomiya's Impossible Demand: "A Cowrie a Swallow Gave Birth To," Part 2, a dismayed and worried Ishigami notes,
"Damn it. She looks desperate for male attention."
Further proof of both Ino being distraught over Ishigami courting someone else and of her having romantic feeling for Ishigami is in the same chapter, when she slurs these words:
Why are you alwaysh... so mean to me? You don't talk like that to anyone elshe... You always, always, always look down on me! I hate you sho much! But you're nice to other people! Be nice to me... like you were at the culture festival... The way you're nice to Tsubame...
Here, she explicitly names Tsubame and indicates jealousy of the way Ishigami treats her compared to Tsubame. This would seem to indicate that she probably likes Ishigami herself, and that she's feeling out of sorts because of Ishigami courting Tsubame.