Although there are weird aspects of the Flashfterlife Universe, I don't think that it was a fantasy or that it was meaningless.
First of all, I don't think it's a fantasy because it was real to the characters. What I mean is, it was something that they were really experiencing after their deaths. Now, could they die in this world too? Probably not. Maybe that's what makes it less real. But it's still a place where interactions are clearly really happening.
Secondly, the Flashfterlife stories throughout season six resolve, for the most part, very important thematic issues these characters have dealt with since the beginning. I'll give you an example by telling you the story of Jack, the way I've interpreted it:
Jack is born to a surgeon dad who is very hard on his son. He grows up being told that he doesn't have what it takes, that he'll never be a good leader, basically that he's not good enough. This, understandably, leads to some pretty severe father issues that follow him for the rest of his life. He also has a great need to prove himself by fixing everything that he sees is wrong in his life, his friends' lives, his family's lives.
Jack becomes a surgeon and marries. He gets wrapped up in his job, probably because he's still trying to prove to his Dad that he "has what it takes." He loses the marriage and begins to fall into the same alcoholism that his Dad is entrenched in.
Christian dies and Jack goes to find him, haunted by the fact that things were never resolved between him and his Dad, especially after he got his Dad fired for being drunk while surgery.
Jack crashes on the Island and proceeds to continue being a control freak, both trying to not be the leader but also unable to keep himself from becoming one, still trying to prove himself capable of such a task. He gets a small group off of the Island, but this doesn't fix his life nor their lives. He gets everyone back to the Island and tries to set off a nuclear bomb in the ultimate fix. This doesn't work and kills a close friend.
Jack finally learns to let go and steps into his role as Jacob, sacrificing himself so his friends can get away. Jack dies.
But he dies still unresolved on his father issues. He still has deep seated insecurities on how he would perform as a Dad and is haunted by the various relational failures in his life. He has a son in this life. A son who he learns to connect with, and they begin to develop a solid bond. The last thing that was missing in his life is resolved.
Jack moves on and lets go.
The same character resolution can be seen with Ben (reversing his choice and saving Alex metaphorically, probably the biggest thing he continued to regret throughout his life as Hurley's number two), Sawyer (destined to be alone, as his speech to Kate in What Kate Does demonstrates, only to find Juliet again in the afterlife), Desmond (finally getting a taste of Widmore's approval, realizing he's already an honourable, self-confident man who has the love of a beautiful woman and giving up Widmore for good), and so on.
