(I know there is already two answers but I will focus more on his flaws as a character, rather than his actions. In many ways these issues with characther is what makes him commit these actions).
Ash is not liked because he have no likeable qualities. He have no personality, no difficulties to overcome or faults you can appreciate. He is nothing more than a avatar for people to project themselves onto. You aren't meant to like him, Ash is someone you want to be. That is the short version.
Here comes the long version:
What is Ash's best quality? What about him makes you like him or want to be him? In basic character building, the worst possible things, his entourage. His partners are funny and interesting (by Pokemon standards) and his Pokemon does all the battles for him. If we were to remove all supporting characters around him he would be completely uninteresting, because he have nothing that distinguishes him. He have no personality on his own and that is the worst sin that any character can commit. If you want another example of this look at Mary Jane from the first Spider Man movies. She's not a character, she's Spider-Man's girlfriend, something for the hero to save, something for the hero to work for, a thing only made to make the hero look good. Ash is only object to center the plot around, nothing else.
Secondly, Ash lacks obstacles and goals. He wants to be a Pokemon master, sure. He wants to catch them all, also sure. But so little focus is out in these and the story progresses at such a glacial speed, you lose interest. That would have been fine, if Ash weren't fine with the slow pace. He never hurries up or actively try to reach his goal, and if the hero isn't interested in his own quest, why should you be interested? Furthermore, there is never anything standing in Ash way. Ok we have Team Rocket and every random bloke with a new Pokemon between Pallet Town and the Indigo Plateau. But Team Rocket isn't a threat, not after the forty second time they tried to catch Pikachu and failed. The Gym Leaders aren't threats, give them a damned PokeBlock or save their precious Ponyta and they'll shower you in badges. A character that faces no difficulty will never have to change or develop, meaning he will eventually become stale and boring.
Thirdly, Ash is too perfect. That may sound very strange, but similarly to how we can't find and good personality trait about him we can't find any bad about him either. You'd think that would make him a better character, but it's really the opposite. If we look at other shows, TV or any other medias the villain is most often the most interesting character. Look at The Joker from Batman, Loki from the Marvel Universe, Fire Prince Zuko from Avatar, heck, look at Team Rocket! Their escapades are far more interesting that whatever Ash has ever done, and their chemistry is spot on. They have very clearly defined characters with personalities, unyielding goals and good and bad within them. Now I'm not saying Ash should be a villain, but even heroes need to have bad qualities to make them interesting and dynamic.
The flaws of the character is what make them interesting, but importantly, it's the flaws that make them human. A human character is relatable, and interesting. You have to put a lot of work behind a "flawless" character and keep it interesting, something Pokemon have failed miserably at.
So what we have is a character with no qualities in his own, goals he doesn't care about and no obstacles hindering him, with no flaws or humanity to keep him interesting. In short, he's a bad character, nothing more.
Sources: My knowledge of basic charcther building, sprung from shows like Fullmetal Alchemist: Brotherhood and Avatar: The Last Airbender, books like Lord of the Rings and Game of Thrones, movies like Blade Runner and the Marvel cinematic universe, a sprinkle of common sense, youtubers like Yahtzee from The Escapist, but most importantly, the Nostalgia Critic and his reviews who will be the only one who gets a link because I'm a lazy bugger that way.
http://en.m.wikipedia.org/wiki/Nostalgia_Critic
