Pikachu terrible performance at start of DP when battling Paul or Roark was clear indicator how he didnt retained power and experience he had in Battle Frontier when battling Brandon.
True in BW Ash was toned down more than it was case in previous regions but i dont think he is back to Kanto persona either being complete exaggeration.Unova Ash is far better at battling than he was in Kanto days and writers showed us in some battles(referring to Elisa and Burgh) and how all of his experience he had before contrary to some belief isnt gone.Personality wise he isnt brash and impulsive in Best Wishes nearly as much as he was in original series either acting more calm and collected to Iris and Trip provocations having more mature approach.
Ash nowdays seem like some weird combination of Johto and Hoenn Ash showing bits of maturity and experience and i suspect highly like i said before how different environment and characters he was put in are main reason behind this.
Ash was part of 5 group combinations since he started traveling with his character interacting differently with all of them with his behavior depending on how much traveling companions are mature.
BW games experiencing revamp along with trying to portray Unova as foreign and far away region also contributed to Ash being put in role of foreigner who hardly knows anything about new region being used as mask to justify toning down.
So in nutshell while Ash seems more inexperienced than he was in Sinnoh,thats more cause of different cast which produces unfriendly dynamic and portrayal of Unova as isolated region with character lack of knowledge being used as way to emphasize on being foreign than anything else.