Sure you COULD, but that doesn’t make it a bad ending. The life after death limbo has been the entire flash sideways for the entire season, we just didn’t know that’s what it was until the end.
Only 3 total “ghosts” were Smoke in disguise: MIB, Jack’s dad on the Island, and Locke on the Island for seasons 5 and 6. All the other “ghosts” were real dead people talking to Hurley or Miles. Though there are indications that the occasional whispers in the jungle were also ghosts, just unable to fully manifest possibly because of who they were trying to talk to.
What about Ben’s mother? Smoke monster or ghost? What about Mr. Eco’s brother? Smoke monster or ghost? What about Hurley’s imaginary friend? Smoke monster or delusion or ghost?