I don't see why you're complaining about this. Obi-Wan isn't a tinkerer, and may just end up making the ship worse if he tried to fix it. Ahsoka (being Anakin's apprentice) has some skill in repairing various electronics, which is why she was working on the ship. It was in their best interest to get off the planet ASAP, so of course Obi-Wan is going to be a little annoyed if he walked in and sees Ahsoka, who was told to repair the ship just sitting around. Besides, you even said about the tinkerers in real life how they don't like people helping. If Ahsoka was working on the ship, why would she have wanted another person (Obi-Wan) helping?