Honestly Finn felt not just wasted but actively shoved aside. I think most of what's needed to be said has been said, and by more amusing people than me. HISHE especially - and when a twelve minute parody cartoon does a story more justice and has better writing than full feature films there is a serious problem with the films.
I don't know if anyone else feels this way, but to me how they handled Ackbar just seemed emblematic of the entire sequel trilogy. Here we have a minor, one note character from the originals. Most of the public wouldn't know him from a Salmon and that's fine. He's unimportant in the grand scheme of things, but most casual fans kind of like him for the memes. To dedicated hardcore fans he's more than that, and by and large they respect his character. In the old EU the character is fleshed out, lives, loves, laughs and in the end is given a dignified funeral.
This character was brought in on purpose. Staff at Disney knew what he was about and in general knew he was well liked.
"Here, see that fish face? Remember him? Remember how he's made you laugh?" goes the film. I imagine many a fan caught on and wondered where they would take this funny meme character. Would Disney take this moment to flesh out their new trilogy? Would they take this chance to weave in some of the better old lore?
"BOOM! HEADSHOT! WITH A MISSILE! Feel how high the stakes are yet?"
I just cannot fathom what the hell the thought process was.