Parables are part of the creation -- there were designed so that people constantly have the truth in their face. The parable of the sower and the seed is one of many such parables.
The parable of the Mustard seed is a confirmation of Daniel 2 (the Statue) and the Rock that was made without hands (Jesus Christ) that will crush all earthly kingdoms and fill the earth. This is happening now -- it doesn't matter how bad it looks -- His Kingdom is growing.