Blerg. My mother used to say this to me all the time when I was growing up.
I suppose it's one of the reasons why I used to (and sometimes still do) fear intimacy. When someone tells me "I love you," I mentally substitute in "I don't like you." It's like in my inner child's mind, the two sentiments are interchangeable and the closer someone comes the more likely they are to hurt me.
Other NM favorites were: "You're so ridiculous, but I love you!;" "You're crazier than a loon, but I love you!;" "You suck. Just kidding! You know I love you." The list of horrendous mixed message-y bullsh*t goes on.
I suppose maybe when the N says "I love you, but I don't like you" he or she is being emotionally honest? Like, they're acknowledging "I love the way you make me feel when you're performing the way I want you to, but I don't like you, in specific (I don't know enough about you for that or care to find out)? I don't know. I hate Ns. I'm feeling so fed up.