So, it's not much of a shocker to find out the movie has a lot of symbolic connection to climate change just from watching five seconds of the film. The whole Tokyo-sinking-into-the-ocean-with-forever-rain is one thing, but there are a lot of little notes, too, like the fact it's a much grimier and shady Tokyo than you tend to see in other animes.
What I think the movie is thematically about is how young people live with it. Almost all the children in the movie are basically struggling to get by in this world with adults who are, at best, benignly interested in them and apathetic. Aside from Hina, you don't ever see anyone older actually try to do something about the rain; if anything, it's presented as this inevitable process that might magically go away every once in a while through some twist of fate. All the kids trying to figure out how to deal with it in the movie are in no way responsible for the actual problem, but they feel that they're the only ones who can fix it.
So the ending of the movie is basically saying, "y'know what, fuck it, the world may be going to hell, but we're kids and we should still be able to sing karaoke and fall in angsty teenage love and...uh...commit gun crimes like all the kids did before us." I have a lot of respect for the Greta Thunbergs of the world, but there's also a level where this tendency to place all our hopes to fix everything on the youngest generation is pretty grotesque--and I have definitely done that myself. So, y'know what? Hina can do whatever the fuck she wants to do. She didn't create this world, and this world didn't do anything for her. The adult hand-waving at the end is a little eye-rolling, but the fact of the matter is that we're handing off a much shittier world to the next generation and we shouldn't begrudge them carving out what happiness they have. So, no, it's not an optimistic ending, but it's an ending that's about finding some small hope in a world without much. So hey, I'm all for it.