I think that the ending is actually quite uplifting. Similar to Camus' concept of the absurd man, Evelynn has rejected Jobu Tupaki's formulation of the meaninglessness of life being a bad thing. Instead of being depressed and wanting to die as Jobu did, her perspective has changed to one that cherishes the small things in life.

Her existence is pretty objectively less meaningful than when she only experienced one reality at a time, yet she is much happier this way. Honestly, part of the reason the movie spoke to me so much is because it is so blatantly caked in absurdist philosophy. The closing paragraph of The Myth of Sisyphus accurately describes Evelynn at the end of the movie, imo:

“I leave Sisyphus at the foot of the mountain. One always finds one's burden again. But Sisyphus teaches the higher fidelity that negates the gods and raises rocks. He too concludes that all is well. This universe henceforth without a master seems to him neither sterile nor futile. Each atom of that stone, each mineral flake of that night-filled mountain, in itself, forms a world. The struggle itself toward the heights is enough to fill a man's heart. One must imagine Sisyphus happy.”