When I say unlikable main character, I'm not talking about an anti-hero. I can always get behind an Elphaba going bad or an Ebeneezer Scrooge going good. I'm actually quite fond of an anti hero. But in their case, I know I'm supposed to have doubts about their morality. Since the book is trying to make them a little unlikable, I can role with it.
A character becomes unlikable when we're SUPPOSED to like them, and then we don't. When we are TOLD something about a character but not SHOWN, that character might start to annoy us. If you tell me that your character is 'really a good person' but all I see them doing for the first pages is being nasty, that turns me against them. And when I turn against the main character, its all over for the pages. Because how can I stay with this person for 50,000+ words if I hate them?
I think one of the best examples for me is of Tris from Divergent. I know a lot of people like this book, which is great. I'm glad you'll get a joy out of it that I never will. The problem with the test at the beginning is that we are told that Tris, in addition to being brave, is divergent for selflessness and intelligence. The book does not put its money where its mouth is because she is only selfless and smart when the plot calls for it and other times becomes borderline sociopathic and stupid. I also disliked the character because the books always wanted me to side with her and I often found myself going against her. Obviously a lot of people like Tris because they didn't see this contradiction so they were perfectly fine with liking her.
There's a simple solution to this: show don't tell. Show who your character is. That way there is no question of whether or not I'm supposed to like them. It will make for a smoother journey through your pages and, perhaps, even an exciting one.
But like with Tris, everyone reacts to characters differently. Though I dislike her, a lot of people love her. In the same way, your character may hit home with an agent but fall flat with another. That's the old subjectivity coming into play. You can't make a MC who appeals to everyone. But you can 'show' and not 'tell', to make it a little easier.