I mean, I think Serena’s brief meeting with Ash could be interpreted romantically if one wanted to deeply analyze that scene. At the same time, it really isn’t overtly “shippy” in any way. Misty, Dawn, and Lillie had some poignant goodbyes too. It was vague enough to hint at something, but also not be anything more than two good friends briefly greeting each other.
From a meta POV, it’s pretty reasonable to assume the reason Ash and Serena didn’t interact for longer was because the writers might have wanted to dodge the elephant in the room.
I’d be very surprised if that’s her last hurrah in the show. They said they’ll see each other again. Ash won the Championship, and some unknown amount of time has passed.
Regardless, based on her Journeys episode, I think it’s fair to say that regardless of what happens with her crush, Serena ended up as a good character in her own right.