We only ever see the parents at church and at their house. 85% of the time we see Tony Danza he's sitting down. We just see him as an uber-intense Italian guy, and the mom was wanting grandchildren.
We see the short friend only when out doing "party stuff".
The other friend provides exposition at the beginning of the movie and then just shows up to console Jon for 15 seconds.
Barbara is sexy and demanding. Even her prudity is a form of demandingness.
Julianne Moore is care free. Even when Jon is a dick to her she just ignores it and keeps talking to him.
The church is restricted to just the confessional and how many Hail Marys and Our Fathers JGL has to do.
And the sister just sits there texting and texting and texting until she one time says something insightful.
There are no arcs for these characters. The only character with an arc is Jon. The movie opens with his cares: his body, his pad, his ride, his family, his church, his boys, his girls, his porn. By the end of the movie, Jon's relationship with most of these things is different. He's skeptical on the church. He's being a better friend. He's done with porn. He's not obsessed with girls. He's feeling less pressured by his family (sort of). He tried something new at the gym. Cool.
But. Because the other characters don't have plot arcs...they're little more than props. They support the narrative rather than add to it. Someone might argue: "The film is about Joe! If you focus on the other characters than the movie isn't about Joe!" Ehhhh. Sort of.
The movie becomes thematic. Don Jon is designed not to tell a story but to make a point about expectations shaped by movies. To deliver that point, we have Jon. That doesn't mean we should abandon the arcs for other characters.
Compare Don Jon to Good Will Hunting or American Beauty or Fight Club or even something like A Knight's Tale.
Which movies seem richer? Deeper?
People might like Don Jon better than other movies. But say we were discussing how well those five movies made use of their characters. Our criteria for measuring this? Character arcs. Don Jon loses. If you compare Don Jon to most movies: Don Jon loses. Even to something like Battleship. Obviously, I think Don Jon is a better movie than Battleship, but as crazy as it is to say...Battleship has more character depth than Don Jon.
Don Jon could have done much more to fully explore the facets of "expectations" by creating arcs for its minor characters. Jon's friends could have demonstrated what happens when you refuse to change your expectations. The sister character could have shown the impact men like Jon have on girls that aren't Barbara/Scarlet Johansson. Yes, it dilutes the 2nd level theme "this is all a metaphor for film", but it deepens the 1st level theme of "expectation" by showing the reaction to expectation. We see how Jon relates to the expectation of Barbara, but we don't see how females relate to the expectation of Jon or of Jon-like guys.
In a way, Buffalo '66 is a fully realized version of Don Jon. It makes better use of major and minor arcs, it develops its characters, it has a 1st level theme and a 2nd level. Compare the use of parents in Buffalo '66 to Don Jon. Buffalo gives the parents a narrative with a beginning, middle, and end. Don Jon just has them. One feels sloppier than the other.
The only defense I can think for the "sloppiness" is that Don Jon is arguing for living without expectations, to being open to experiences, to not making things perfect. Which means Barbara is a bit like Buffalo '66 and Julianne Moore's character is Don Jon. Not perfect, but...it can make you happy. In other words, the style of Don Jon mimics the narrative themes. Which is high level stuff. A sort of basic example of this is Memento. The distortions in story telling mimic the distortion of the main guy's memory.
But I still think Don Jon could have done more narrative development while still mimicking the "you need to not have such unrealistic expectations and enjoy what you can enjoy." I understand how that's a bit hypocritical. But is expecting a movie to not be so one-dimensional any worse than expecting someone to shower multiple times a week? Not all expectations are bad. Which is another topic Don Jon could have explored.