Speaking personally, I think one of the difficult things with poetry is attempting to define a meaning, or to project what anyone else will interpret, particularly when the interpretation makes a very broad statement in relationship to the material available. We might also consider what the author originally intended, and the role of authorship in the meaning of a work; if we interviewed this author, and they stated that mindfulness was not the point of this haiku, would it diminish that interpretation or take away meaningfulness?
I think the most direct way we can make a connection between mindfulness is to consider how our individual concept of that word leads us to find such a meaning in the poem. Taking myself for example, my thoughts immediately connect the "mountain road" and the "wild violet" - I envision someone outdoors, in a unpopulated area, seeing a lone flower, and they stop to consider the simple and natural beauty of such a circumstance, not really knowing why they do. However, I can also envision a variety of other interpretations;
- the "tugging at heart" can be seen as homesickness, loneliness, melancholy or some other sort of sorrow that is difficult to express. Perhaps the flower is a reminder of someone, or something, meaningful to the observer.
- The flower may represent nature, and the observer is reflecting upon the impact of humans on the environment.
However, I think the key connection to mindfulness is the word "somehow". The observer doesn't appear to know exactly why the wild violet "tugs" at their heart. For that matter, perhaps it wasn't necessary that it be a wild violet; something about the scene inspires an emotional reaction in the observer. I think there's an important but unmentioned implication in this interpretation, and that is that we seek and enjoy the act of discovery and the revelation of things unknown to us. Thus, mindfulness (being aware of one's surroundings) will lead to moments of desirable "moments" such as these, without even knowing what they will be.
Commentary and some elements of translation are available in the link below.
When people go about their everyday lives, the beauty around them often goes unnoticed. The poem says that the wild violet tugged at their heart because it is a beautiful flower that people, with their busy lives, never stop to realize how beautiful it is. We can apply this to our own lives, because the flower is a symbol for the simple beauty around us. Due to the emotion the author experiences, it encourages us to take the time and experience it ourselves.