It's sort of weird that out of all of Yamanda's friends, the author would have Serina be the one to have fantasies of Yamada and Ichikawa being romantic. Partially because, from my perspective, none of Yamada's friends would fully approve Ichikawa being Yamada's boyfriend since none of them view Ichikawa in high regards, at least at this point in the timeline of the main manga.
If one of the friends would be the supportive one, I would suspect it would more than likely be Chihiro, who I think is at least somewhat aware of Yamada's feelings for Ichikawa. She seems like the most open minded and level headed of the friend group, plus she got along with Ichikawa in chapter 35 of the main manga, but she also appears to think that Ichikawa is too much of a gloomy type (as shown in the same chapter) and is usually creeped out/annoyed by Ichikawa's actions. Moeko, on the other hand, views Ichikawa as a "slime" in terms of the social/dating hierarchy, as seen in chapter 31 of the main manga. As for Serina, with almost every chapter she appears in, she glares at Ichikawa and she generally views Ichikawa as a nuisance, like she seems to view all guys.
Would it be a nice juxtaposition to see the tomboyish girl have a secret hopeless romantic side, sure. However nothing in the main manga, so far, supports Serina as being that kind of character, let alone have her approve of Ichikawa being with Yamada.