My child can say "papa" but not "mama." Does she dislike me?

No — it has nothing to do with feelings; it's a difference in sound structure. "Pa" is a sound made by closing the lips, so it can be formed with the lips alone, but "ma" is a sound made by passing air through the nose (a nasal sound). When air doesn't rise easily into the nose, the "m" sounds are hard, and it tends to come out as "baba" or the like. Children with Down syndrome can have trouble passing air through the nose due to the structure of the mouth and nose, and this is said to be the reason.

Even if she can't say "mama," she's simply calling you as her *closest person.* Don't be disheartened — let her hear "mama" plenty. (Note: this explanation of sound structure is a practitioner's view. If you're concerned, consult a speech-language pathologist.)