Well Finn states early on in the film that he lived with his sister Maggie and her boyfriend Joe and that they were poor and scrapped by on what money Joe Coleman could get from odd jobs and fishing charters, so Joe was a poor guy that had raised him, now Joe was a nice and honest hardworking man and raised Finn well but Joe was kind of crude too, swearing when Ragnos and Finn first met and acting behind the times.
Later on once Finn has the chance to go to New York and become a successful artist he takes the opportunity and runs with it, if he had stayed in Florida he would have never become a success and broken out of his dead end day to day lifestyle and probably become a tradesman like Joe was, not that there was anything wrong with that but Joe wasted his life and never made any money, he was born in Cortez Fishing Village in Manatee County near Bradenton and would probably die there and never left a mark on the world at all.
Finn on the other hand used his talents as an artist to break away from the dirt poor small town Redneck lifestyle that Joe had embraced, in a way Joe was holding him back in Florida and molding him to become just like how Joe had always been, another day to day worker with no future prospects and struggling to make money to survive.
So Finn rejected Joe after he got to New York and started making money, he even made the story up about how Joe was a drug smuggler and had died on a drug overdose, then Joe shows up at his gallery show and humiliates him and makes a fool out of himself, Finn looks like a liar and his past as a poor Redneck fisherman is revealed when he never wanted anyone to know about where he came from, granted he should have told the truth but he does not, he lied and Joe by showing up brought it out in the open and made them both look bad.
I dont see Joe as anyone to be ashamed of, but at the same time Joe is a homely looking and acting Redneck fisherman, doesnt have a lot of class, swears and acts poorly in front of Finns New York friends, Joe just doesnt belong in New York but he wanted to support Finn, after he makes a fool of himself he tries to leave very quickly, he succeeds in making Finn look bad too and like a liar because he claimed that Joe was drug dealer who had O'D and he had not, thats not something that will impress anyone at an art gallery show lol.

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Whaa?????????????????????!!!!!!! Chris Cooper is sterling. Again. He's really a talent.

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Whether there was ample reason or not, the scene stayed true to the book in some way. I haven't read it in a while, but I seem to recall a scene in London in which Joe "popped in" on Pip and his more proper roomate. Joe displays poor table manners and a crude use of a knife, embarassing Pip in front of his new colleagues. That is why this happens in the film, I think.
It's Gwyneth's head in the box.

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Yeah, that's the thing. Nothing was wrong with Uncle Joe. He was an honest, hardworking man who trained his orphan nephew in a living trade.
All of a sudden, when Finn "made" he was all of a sudden ashamed. I'm glad Finn had the grace to be embarrassed by his behavior in the scene outside of the galery.
No two persons ever watch the same movie.

What the character of Finn failed to realize was that his story was already a masterpeice; however, he was too blinded by his obsessions (as one of the previous posters had before mentioned) and his self-inflicted shame to appreciate how beautiful and intriguing the story of his life truly was.
He was always running away from what was truly golden and authentic and chased down dead ends and red herrings.
In the end, everyone learned something about themselves (Finn, Estella, Dinsmoor) but at an unfortunate sacrifice, to each their own.

I always took him shunning Joe at the party because of he was ashamed of him. Not ashamed because he raised him, but ashamed that he, himself had roots as a poor, redneck fisherman from the wrong side of the tracks.
It's funny too, because when he's first getting his loft he mentions something about being a commercial fisherman from the Gulf. And he tells Ragno that someone "wants to turn this frog into a prince." So it's like he liked it at first, the attention, everything. And the following scene and short monologue show that.
But after being around and being part of that world that he first so longed to be in, seeing anything from his life made him ashamed, as if he never grew up like that in the first place.

On the way up in the art world, Finn was trying to be someone that he wasn't. He was trying to be more "tortured" and interesting because that would make his art more valuable in some way. He was just being an beep because it would sell more art. A lot of teens and adults shun their roots to sound more interesting to the world. Who wants to hear about a perfect childhood with a perfect family?

The fact that Joe was always good to him is what makes Finn's actions cruel. He's obsession blinds him.