Misty is right to get angry with Ash some of the time, especially since he did destroy her bike which probably costs a lot of money in the Pokémon world. Also, Ash can be a jerk to her too.
When she and Ash get into an argument when the latter mopes about his loss at the Indigo League, Misty makes a good argument that Ash should have been lucky to have even made it far in the Indigo League with the way he was training. She’s right, considering how lazy Ash became and only relied on luck to make it through. This contributes to some major Character Development for Ash, as he starts taking his training much more seriously and places higher in future league tournaments. He even eventually becomes the Alola League Champion, and later the World Champion.
Ash, during his more immature phase, also gets his share of these:
While he wasn't being nice to Misty, he pointed out that he was attempting to be nice—she was the one refusing to let the fact he accidentally fried her bike (expensive or not) go and being mean to him for something he promised to rectify.
Likewise, when he mortifyingly lost the Indigo League, he calls Misty a Hypocrite for chewing him out for his poor performance, as she could be just as haughty, lazy, and vain as he could be.
Paul gets quite a few instances.
The thing that Paul most often gets on Ash's case about is his blind reliance on The Power of Friendship, to the point where he actively chooses it over strategy or rational thought (far too often does Ash persist against blatantly outmatched odds solely because he believes in himself). A large part of Ash's development in Sinnoh is learning to balance his beliefs with Paul's.
Paul also eggs Ash on about training Chimchar to master its Blaze ability, which Ash refuses to do knowing the trauma associated with it. While Ash is just looking out for Chimchar's health, Paul's comments about Blaze aren't exactly unfounded either: as Chimchar goes completely berserk when the ability is activated, there's no way Ash will reach its full potential without getting that power under control. By the end of the series, Ash does essentially validate this belief by using a now-mastered Blaze to defeat Paul, albeit mastering it through The Power of Friendship rather than Paul's Training from Hell.
He rudely insults Maylene after curb-stomping her in battle, calling her the weakest Gym Leader he'd ever fought. Cruel as it was, though, Maylene herself admitted he was right — she and her Pokémon were completely out of sync.
Barry keeps praising the Sunyshore Gym as one of the best which Paul criticizes as undeserving. It's later revealed the Sunyshore Gym Leader gave badges without a challenge of any sort. Ash effectively agrees with Paul by demanding a proper battle.
When he meets Ash at the Sinnoh League, Ash claims he'll be giving it all he's got in their battle. Paul states that everyone participating has gotten all 8 badges too, so going all out against him and the other trainers Ash is battling amounts next to nothing. Even Ash admitted that Paul was right.
During the Sinnoh League episodes, he also tells Barry that overestimating his abilities will only backfire on him. Paul is proven right when Barry gets curb-stomped HARD by Paul.
In Best Wishes, in the first round of the Junior Cup tournament, Fiery Redhead Georgia and her Beartic suffer a Curb-Stomp Battle at the hands of Iris and her exceptionally powerful Dragonite, which had decided to join Iris' team of his own accord just in the previous episode. Georgia tells Iris that she didn't actually win the battle, but that Dragonite had won all on his own, as Dragonite wasn't obeying her at all. While Georgia's often a Sore Loser, her argument makes perfect sense — that instead of relying on Pokémon that she trained and fought alongside, she's just using a last-minute super-weapon she just found. It also helps that a battle should demonstrate a bond between trainer and Pokémon. It comes to a head when she battles Ash; she and Dragonite seem to finally be working together until Ash's Krokorok evolves into Krookodile and gains the upper hand. Dragonite starts disobeying again and goes on a bit of a rampage which leads into an embarrassing loss for Iris, which makes Georgia extremely pleased to have been proven right. Iris acknowledges this at the end of the tournament and promises Georgia that she'll have Dragonite under control by the next time they meet, so that Georgia can have the battle she wants.
Despite being condescending, Iris has a few points:
Iris gets flak for calling Ash a kid, but she's right to criticize him sometimes when he acts very childish (for example, Ash purposely using a Pokemon against another Pokemon that has a type advantage over his ...it helps that other characters such as Paul and Brock have called Ash out on this habit too). Even Cilan took her side one time.
In the episode "Evolution With Fire," she criticizes Ash's Tepig for being too nice regarding its former jerkass of a trainer. Iris is proven right when Snivy tries to verbally talk some sense into Tepig and also by using Vine Whip on it.
