What do you think about me telling my friends she's not so smart and also imply that she's not a very good friend.
What is making you want to say that to your friend?
She's not that good of a friend.
I think if you two are having some problems you should own up to your true feelings.  That being said, you need to do so in a way that doesn't just put her down.  The conversation will go much more smoothly if you don't resort to insults.
I just mean what I say. I don't have to explain everything to you.
It's important to be respectfully honest with your friends. However, be sure to consider what your goal is and whether or not what you're going to say will help or hurt your friendship.