In thinking a little about this, we could say the fruit (which we often call the end result) doesn't even matter. The condition of the fruit is irrelevant (is the end result 'good' or 'not good'). Even if you are able to give attention to the action itself while you are in the action, please don't get to the end of the action and then judge the fruit. The action can BE the fruit.
The act of doing Zentangle is the fruit. No matter what you are experiencing as you do your Zentangle, that is the gift. Whether it be joy or frustration, All of it will show you what is in the moment. No wrong, no right. It's just a moment.