"Regression" is the wrong word. Ash Ketchum is "flanderized" into an exaggeratedly one-dimensional hyper-active idiotic children.
Was he such a hyper-active idiotic character back at OS? Well, I wouldn't completely rule it out, because he was indeed an academic idiot shown at The Ultimate Test back at then, and he was indeed quite playful like to tease the others especially Misty such as during The Tower of Terror and likes to enjoy playing such as during The Pi-kahuna. But are these his defining traits that we saw most of the time? No. I can still remember during the OS, the most memorable character traits of Ash Ketchum is his seriousness and courageousness specifically towards Pokemon, whenever his or any other pokemons he cares are in danger he wouldn't think much but just stick his body at the front to protect the others, he always placed pokemon being the first thing on his priority list (Ah..... possibly his stomach is more of a priority, IIRC). He was immature and especially quite haughty, where whenever someone mocked him his shortage or weakness, he becomes upset and will try his hardest to disprove the person. To be fair no need to wait until the XY saga's Mary Sueish Ash, his haughtiness was dropped during OI, so Ash getting upset at being mocked had become out of the context.
But during SM, where were all of those most memorable most defining character traits of OS Ash? No there isn't, I only saw the most minor traits of past Ashes being overly exaggerated, and on the opposite the previously most significant characteristics become the minor. That is the SM Ash I see, a severely flanderized Ash Ketchum.
And that says, I give my two cents vote to the slapstick comedy part as well. It is overly exaggerated to the point of creating opposite effect. Rather then funny, I feel utterly obnoxious whenever I saw anyone pulls off a parodistic facial expression. And out of all the characters, Ash Ketchum and TRio are the worst offenders.
Oh yeah, I also forgot to mention TRio. Well, they are not anything I especially dislike, because I had grew to the point of feeling completely indifferent to them. Whether they stay, appear, blasted off (or kidnapped by Bewear), put on a bus, or killed on-screen, I don't have any emotions towards them.