It is definitely derogatory. A 'know-it-all' is someone who irritates other people by showing off their knowledge - this is never a positive quality. If your dictionary defines it as praise, this is a very poor and inaccurate translation. 'Just' and 'little' add to the derogatory nature of the comment.

Generally it is not a compliment - it means someone who acts as if he knows everything and is showing off. However it can be used in a friendly way between friends - if one friend tells the other something she did not know before, she can gently say to the other ' oh, you are a little Miss know it all, aren't you ??

That's such a funny word, but know-it-all people are very annoying. There used to be a few of this type on Italki, especially in the discussions.