If someone is giving to the poor, that's good.  That's what God wants us to do.  But, if someone is giving to the poor, but losing sight of Jesus in that action, that's sinful.  The action is being done with the wrong intent.

Jesus considered this woman's desire to please Him more important than giving to the poor because Jesus is more important than the poor and should be considered first.  If she would have just given to the poor, while neglecting Jesus, her actions would have branded her as unfaithful.