From the wikia you can see that he always was intented to be more immature than the others:
Ron was a very funny person, but often emotionally immature, and insensitive. He had a sardonic, wisecracking sense of humour that often brought his friends laughter and relaxation. However, as Hermione Granger and Luna Lovegood pointed out, his humour could occasionally be hurtful, even if unintentionally. Hermione was frequently frustrated with Ron's immaturity, once commenting that he had the "emotional range of a teaspoon". During his time searching for Voldemort's Horcruxes with Harry and Hermione, Ron matured greatly, and even took to leading the trio when Harry temporarily fell into a depressive mood.
In a pottermore article about Ron:
He may have brought the much-needed comic relief, but he also brought so much more.
To begin with, Ron was an occasionally insensitive, insecure and mildly jealous 11-year-old. By the end of the story, he’d matured into a man who knew his own worth, stuck up for those who didn’t have a voice, and learnt that he had no reason to be jealous of Harry.
Standing next to Harry and Hermione, it’s easy to see how Ron might have felt underestimated; Harry was often seen as the brave one, Hermione as the clever one, and Ron as the funny one.
Here's an article where JKR speaks about marriage:
The truth is that in my younger days, I dated Ron more than once. Ron is funny, very funny; he's insensitive. There's a lot of immaturity about Ron, and that's where a lot of the humour comes from.
Here's another interview (translation of dutch interview):
Who would you rather have as a son, Harry or Ron? “I’ll take them both! I adore Ron. Ron is the most immature of the three main characters, but in part seven he grows up. He was never strong footed, people see him mostly as Harry’s friend; his mother had actually wanted a girl and in the last book he finally has to acknowledge his weaknesses. But it’s exactly that which makes Ron a man. The others also have such a moment where they truly grow up. Harry when he reacts so fiercely against his former professor Lupin who decides to abandon his family. Hermione when she is forced to choose between Harry and Ron. Hermione never strays of the path; she always keeps her attention focused on the job that must be done.
And he was really insecure about his abilities and he was kind of hard in himself because:
Ron was also prone to insecurity about his abilities, when compared to his older siblings and to his famous best friend. When he felt insecure or embarrassed, it was noted that his ears turned red. This became his tell-tale sign of his anger and embarrassment. He felt as if he was consistently overshadowed by the legacy of his older brothers and the fame of Harry; this lead to a feeling of being "second-best".
He was the second-youngest child in a whopping-great family, and the least academically accomplished of his friendship group (yup, Harry got better marks even if they both copied Hermione). Plus, with the whole ‘my best mate is the Boy Who Lived’ deal, life could sometimes seem a bit of a bummer to Ron.
His inner struggle, as Dumbledore explained, was revealed by the Mirror of Erised The usually-cheeky Ron’s true insecurities about the achievements of his siblings were revealed when he looked into the one place that showed your heart’s desires – and saw himself, a success, and standing alone.
Fred and George mocking him didn't help either. They stopped only when he started playing quidditch.
He was also lazy:
Ron was also a person who liked to do things the easy way, as he did not like to work hard. This quality was reflected in his first year when he just picked up books from the Hogwarts Library shelves randomly when finding out about Nicolas Flamel. He also had a tendency to make up predictions for his Divination homework, and frequently copied most of his homework from Hermione Granger.