I'm going to have to totally disagree with what you said about the Battle Frontier. If anything, that was Ash's peak. Ash used plenty of his old Pokemon, had some great strategies, and eventually defeated 7 amazing trainers.
I feel like Ash was improving from when he first got Pikachu until Sinnoh started. Ash felt like a total failure in Sinnoh but eventually got a random spurt of awesome during the league.
Now Ash doesn't know anything, and he's losing way too many battles for someone with his experience. If the anime was realistic at all there would be no reason fro Pikachu to beat Snivy. Seriously, it doesn't matter if Pikachu had not electric moves. Why would you use electric moves on a grass-type anyways? Yeah, I understand this is all for plot but it still unnerves me.
In regards to Battle Frontier, I never said he wasn't good. Just not the same as Kanto, Johto, or Hoenn. It was different and his battle style changed slightly. I definitely agree with you that he was good and defeated some extremely strong trainers, Ash was just different. Not in a sense of experience, just different than the previous seasons. I can't really explain it at the moment (maybe I'll find the right words after thinking about it later).
Ash did seem to be stagnant with progress when first arriving at Sinnoh. But I actually loved how they made him progress through the region (even though it was disgustingly long) because that was where we were given the first glimpse into Ash's true battle style. I mean the "Counter-shield" move was really original and the more he battled, the more this style was shown. I saw him progress starting from about the point he gained Chimchar all the way to when he faced Tobias.
Ash actually didn't make a "rookie mistake" or anything of that nature with the whole electric vs grass type thing. Pikachu is strong, we know it and so does Ash. And he also knew that Trip's Snivy was new and not that strong as of yet. If Ash could have used Pikachu's power as demonstrated against Tobias and Latios, Snivy would have been BBQ. And Pikachu has demonstrated plenty of times that types are not everything. Ash doesn't battle that way (he has said so many times himself) and Pikachu's Thunderbolt most likely would have ended the battle if Zekrom didn't block Pikachu's electricity. We still don't know if Pikachu's power levels are still even the same (Juniper said it was alright but that could have been directed towards it's health more than power levels).
Writing all of this, I think Ash remembers more than we might think. It's just the evolution of his character, battling style, change of friends, and the people around him that make it less noticeable. Sure he has his slip ups, but I mean we all do. With Unova, the writers just handled the who "tutorial" part badly as they just as easily could have made it like Pokemon Advance with Ash teaching May (or in this case Iris) or even a new trainer aside from Trip.
And with the whole Zekrom thing, I personally believe there is more to what happened. It just seems unusual to me that they would only have that event occur to weaken Pikachu. We'll see as the series progresses, but I believe there is a reason as to why Ash lost to Snivy/Trip and why Zekrom blocked Pikachu's electricity. But that is probably for another thread...
By the way, I hope you don't see this long response as me trying to argue on some topics. Just having some conversation sharing ideas.