It can actually be used either way -- a lot depends on the tone of voice. You can say "He's a smooth talker." (and he has completely made me fall in love with him) or "He's a smooth talker." (so you'd better watch out, because it's all insincere flirting.) The same with "suave". These words literally convey a positive meaning, but they are used very often to mean just the opposite. Tone of voice and context are crucial here!

When someone is "suave" or a "smooth talker" it means they are charming and persuasive. It can be positive or negative depending on context. Positive: If for example a girl is talking to the guy directly and says "you're so suave!" then it's positive. Negative: If for example one girl is telling another girl "be careful with him, he's a smooth talker/suave" then it could be that she thinks that attribute is negative. Maybe the guy will persuade her to do something bad.