I would say that Ash is a disappointment if you look at his character as a whole, because he never learns anything. It's like trying to hold water in a leaky bucket. Everything he goes through just drips out over time and he's left an empty shell by the time the next season rolls around...obviously he needs to be this way, because he's been at this for a long time now and he'd be Champion-level if he retained everything he'd learned, but even so.
But on a series-by-series basis, Ash's competency is variable. Personally I only found him a disappointment in BW and XYZ, because his league losses were so badly written that it ruined the entire arc. Whilst I like DP Ash the most, I would say Battle Frontier Ash and Orange Islands Ash are the two least disappointing, because they actually accomplish what they set out to do.