And those would have made for better endings, yes.
To be clear though, it's not just "Finn wasn't enough of a hero" that made the ending shitty.
We'd gotten whole eps devoted to him feeling bad about not having a girlfriend, and it ended with a wishy-washy "he might be dating, probably not serious" relationship with Huntress Wizard.
And after fucking SEASONS to think of a way to resolve Simon and Betty, we just flipped the script, with Betty now magically trapped similar to the way Simon was. Wow, no resolution. They had years to come up with that.
And GOLB was a shitty out-of-nowhere final threat, with no style, no personality, and so poorly explained that I guarantee you there were shitloads of people looking him up online that day going "wait, WHO?"
We've had much worse endings, but it was still a piss-poor ending.