Jango Fett was most definitely not inept. At least, he was not intended to be, at the time.
As another answer has brought up, Jango was used as the template for the clone army. While this does not necessarily shed light on his own ability, the means in which he was chosen certainly does. The story is told through the video game, "Star Wars: Bounty Hunter"; it certainly does not constitute canon, these days, but it was written and developed by Lucas Arts at the same time as Attack of the Clones; and thus it gives insight into the author's intentions, regarding the character.
In order to be chosen as the template for the clones, Jango Fett has to complete a particularly challenging bounty task, set up as a 'competition' of sorts among several bounty hunters who were considered 'the best'. The penultimate task was to kill a Dark Jedi. In other words, Jango Fett was chosen as a template for the clone army due to his ability, specifically his ability to kill a Jedi. This is certainly not a skill you would equate to 'being inept'.
Fett arrives on Kohlma, the hideout of the Bando Gora. He arrives at Vosa's castle, where he finds Montross waiting for him. They duel a final time with Montross wearing his Mandalorian helmet and jetpack. Jango finally defeats Montross, who wishes to have a warrior's death. Fett, as a means of revenge for his murder of Rozatta, lets the Bando Gora tear Montross to pieces as he walks away. Upon entering the castle, he is taken prisoner. He is tortured both physically and mentally by Vosa. However, Wesell then arrives, but is injured by Vosa. As Vosa moves to kill her, Wesell blasts Jango's restraints, freeing him. Fett follows a fleeing Vosa through the castle and ultimately fatally wounds her. As she lies defeated she is force choked by Darth Tyranus, who then steps from the shadows. Tyranus explains that the entire ordeal was a test, and that Fett has passed with flying colors. He offers Fett a considerably larger sum to go to Kamino, to be the template for a clone army. Fett agrees, on the condition that he gets the first unaltered clone for himself.
- Ending summary for "Star Wars: Bounty Hunter" @ Wikipedia
Again, one might consider the above a moot point, considering this lore has since been disowned. However, it certainly gives us insights into how Lucas Arts were attempting to portray the character.