Reading this again, I noticed some things I initially missed.
https://dynasty-scans.com/chapters/the_blue_star_on_that_day_ano_koro_no_aoi_hoshi_ch04#23
It seems to me like the "Mermaid" that Segawa is talking about here is actually Koumoto.
1) Her talk about the "person I like", and the mermaid being "surrounded by friends", fits much better with Koumoto than the mermaid that supposedly represents her mother.
2) She starts to get emotional after Ibu openly hates on her in front of Koumoto, something we see to be a sore point for Segawa later on.
3) This scenario mirrors how Segawa tried to distance herself from Koumoto after encountering her bullies, feeling she's not good enough for Koumoto
https://dynasty-scans.com/chapters/the_blue_star_on_that_day_ano_koro_no_aoi_hoshi_ch07#26
It really lifts up this later scene for me even more, because Koumoto here is just doing what Segawa was already doing earlier on; pretending she's not talking about her feelings for the person right in front of her.