Yes, Joel starts out the series as a dirtbag who cheats on his wife, and leaves her for his secretary, and is not very good at his job.

But.

Everyone overlooks all of his good qualities, and how much he has grown over the seasons. He always supports Midge's career, and always belives that Suzie knows best when it comes to Midge's career. He always protects Midge, and while she'll say that she doesn't need protection, she has a tendency to get into trouble, and Joel is quick to protects. He always defended Midge, and is always there for her no matter what. He is the only man who is always there for Midge, and has her back. He is the only person who fits into every aspect of her life. Lenny Only fits into the part of her that is a comic, but not the other parts. Lenny is also an addict who won't get better, and dies in 1966, so him and Midge are essentially doomed. He also never Stays in her life for too long. And then there is Benjamin, who wasn't it for Midge. He was amused by her career, but didn't understand it. Joel however, does understand those aspects of Midge that the other men don't.

He becomes a successful business owner, was supportive of Mei being a doctor, and supportive of Midge's career as a comic. He is above everything else, a doting father.

He is a product of his time, but he puts his kids first. He is a more present father than most men would be in that time. The reason he's in prison in the flashforward is probably him protecting Midge, or taking the fall for.

He's not a perfect. He's very flawed, but so is Midge, yet we still root for her.