I don't find it depressing. She spent her life with the person she loved, and got the chance to see gay people finally given the right to marry who they want. I think she was crying tears of both joy and sadness.
Seeing this makes me see more mother x daughter doujins. Call me creepy, but hey that's the most interesting incest tag but there isn't almost anything of this.
so there was an interview about if there is a second season for this show and some things that wont be in for sure is romance, BUT when talking about romance in the show " ....if akko needs that kind of development she already has Ursula and Diana" so there its half cannon :P