He was forced out of his swamp, and discovered that the world had so much more for him to enjoy. He still goes back to his swamp at the end of the day, but with the understanding that he can leave and find other joys whenever he wants, which he does in subsequent movies. Fiona is the main reason he changed so much, yes, but he fell in love before knowing she was also an ogre. Even if she had been human, the story would be the same.
It’s basically the same as a guy living in a trashed apartment being forced by concerned friends to spend some time outside, and learning to live a more well-rounded life, while finding a partner with whom to share his type of lifestyle in the process.