To be fair, I think that's due to two main factors.
The first being the type of show SG1 was: largely self-contained episodic monster-of-the-week. They need their new team member up and running as quick as possible. Remember, this was the early 2000s, TV didn't really lean into the strong ongoing story until roughly early 2010s. And yeah, there were obviously exceptions, but it was rarer for the time, especially in sci-fi.
And secondly is that we did only get him for 1 season. That leaves very little room to develop his character all that much without focusing too much on him. The only main characters that would be comparable would be Cameron and Vala, the latter of which was directly related to a core story point with regards to the Ori (and hence got more attention to flesh out her character), and the former was a bit of a Gary Stu himself, though he had that extra season (and 2 films) to help flesh him out more than Jonas ever got.
To be fair, I think that's due to two main factors.
The first being the type of show SG1 was: largely self-contained episodic monster-of-the-week. They need their new team member up and running as quick as possible. Remember, this was the early 2000s, TV didn't really lean into the strong ongoing story until roughly early 2010s. And yeah, there were obviously exceptions, but it was rarer for the time, especially in sci-fi.
And secondly is that we did only get him for 1 season. That leaves very little room to develop his character all that much without focusing too much on him. The only main characters that would be comparable would be Cameron and Vala, the latter of which was directly related to a core story point with regards to the Ori (and hence got more attention to flesh out her character), and the former was a bit of a Gary Stu himself, though he had that extra season (and 2 films) to help flesh him out more than Jonas ever got.