I really don't have a problem with Luke being an idiot since the show itself knows he's an idiot. Problem is when a show has an idiot for a main character, and the show thinks the character is cool. Kinda like the main Gundam SEED characters.
Show knows Luke is an idiot, they'll develop him. This is why it's a bit tough in the entertainment business. It's very hard to develop a character outside the usual mold (the cool guy). There's a reason for it, they are usually annoying. In my opinion it's fine. It's not the usual main character we encounter in these kind of series, and as long as the show itself knows how moronic the character is, and develops the story accordingly it should be fine. And for the last couple of episodes, I think they have done a great job with Luke (mainly in how the characters around him are interacting with him).
But whatever, to each his own. We don't have to like them all. But, it's not that I like Luke, in fact I hate him... but I don't think he's a bad main character. Is it really necessary for us to like the main character? Wouldn't it be possible for once to enjoy a show that has a character we're supposed to dislike?