Nanako is a bitch, but that doesn't mean she'd have been wrong when she called Kageyama a loser. He's a bona fide one. Based on earlier conversations, he's giving the songs to Natsukawa for free, so that she can build her career more efficiently. Having catchy, high quality original songs is actually an astronomical boost for her. "Anyone" can sing covers, but exactly how many people can present competitive original songs one after another? That's not a mere idol anymore, that's an artist. A fledgling idol wannabe wouldn't have the financies to get those from a professional, unless a big idol agency had taken her under its wing, but that comes with a contract and obligations.
Yet Kageyama gets nothing in return. No money, actually not even reputation: Natsukawa didn't mention after the show who was the song author. No other kinds of benefits (that you could imagine if you wanted to). He even got treated like dirt in this chapter. After that humiliation, he got a half-assed apology. He would have been better off without that so called apology. Clearly Nanako the bitch believes they are now cool and if Kageyama has a problem with it, he's the bitch.
The guy needs a friend who trains him not to be a loser. Not gonna happen in this scatterbrained story, though.