Just to add onto what
@leonleon said based on what I've read from the Japanese raw:
The 'dumping' part is because Yoshino is going to check out universities specifically in Osaka. Kirishima asks her if she's going to take an entrance exam for a university in Osaka, and she goes, 'Of course, from the looks of it (or for now), the chances of me coming back to Osaka are high.' He jokes around saying is he being dumped right now, and she says, if he calls this dumping, then wouldn't that mean she's been dumping him since forever? Yoshino also mentions how if he goes overseas, she can introduce to him a lot of good people, and mentions a few places where she has acquaintances. He's sort of surprised she knows a lot of people (his actual words are 'I've thought this since yesterday, but you have a lot of acquaintances, huh'), and Yoshino brushes it off by saying that's to be expected since it's her hometown and she directs the question at him rhetorically by saying, 'I mean, you know a lot of people in Tokyo as well, right?'
I actually don't know if he's bummed out about her having a lot of acquaintances because: a)he's jealous, b)he doesn't know much about her, c)because his 'acquaintances' aren't exactly his acquaintances, or d)she's going to leave him (having tons of acquaintances in Osaka + looking at Osaka universities), but as much as I hate his two-timing, I actually enjoy the fact that Yoshino doesn't get bummed out about his cheating because a guy who two-times doesn't deserve her love. Honestly, the feeling I'm getting from them is Yoshino friendzoning Kirishima, especially from what's been summarized in the previous chapters (where Yoshino thinks he's misinterpreting his feelings as love when it's just being happy at making a friend).