Pretty much this. 4 times top 8 in a tournament with over 200 compeditors is pretty impressive. Although he wears plot armour for most of the time, he does very well, with good tactics and good matches.The way I look at it, imagine you're in the anime version of the Pokemon world. There is one Pokemon league per region, per year and each is entered by a large number of Pokemon trainers (at least 256 in the Indigo Conference). Ash has finished in the top 16, top 8, top 8, top 4 and top 8 so far. In my opinion, that is nothing to be sniffed at. Assuming each has a similar number of trainers competing, he has finished in the top 10 out of about 250-ish four times, which I think is fairly impressive. In fact, during the Lily of the Valley Conference, had it not been for a trainer with at least two legendaries, there is a fair chance he would've won it.
Looking at it that way, I think I do believe in Ash Ketchum as he is always there or thereabouts. No denying Ash is a bit of an 'also-ran' though. The problem is though, as stated by many people many times, the problem with the series is that it is written with the assumption that with each new generation of Pokemon comes a new generation of fans, and this meant that he anime isn't progressive and Ash has been 10 since forever and doesn't seem to have really improved. Despite his impressive league finishings, the fact that Ash doesn't seem to learn and improve very much really takes away from his credibility.