The performances are fine, the directing is OK, and the overall premise is strong, but this movie never finds its tone, and the writing just isn't compelling. Given the premise, what should work here is humor, but the plot is saddled with too much emotional weight for the director to be reasonably expected to wring humor out of Hesher's intrusion onto this unassuming suburban family. It's hard to get a laugh out of a flaming diving board and subsequent VD confession when they're being forced upon a child who is already dealing with a serious familial trauma and a despondent, grieving father.
And that's where the writing fails: for most of this movie, I didn't really see Hesher and TJ's dad as really all that different, because it's not until the last 10 minutes of the movie that the viewer gets a glimpse into TJ's life before his mom's death. Having never assumed that TJ had previously been living in a kind of Leave it to Beaver fantasy world, I had no grasp on just how much his life had changed since her death -- I could only tell that he and his father were very sad and angry. Moreover, Hesher just seemed like another in a string of oddities in an otherwise decentralized life. That means that rather than feeling a sense of momentum and culmination as the movie climaxes, I was left wondering why the movie's close was presented with all these grandiose emotions I just wasn't feeling and why I hadn't laughed more.
Ultimately, this movie would have been stronger if the writer hadn't tried to cram all this self-serious, hyperemotional content into it or if the writing created clearer distinctions between TJ's life before and after Hesher's arrival.