If mindfulness is conscious awareness and nonjudgmental acceptance of moment-to-moment experiences, mindlessness is the absence of such awareness and acceptance (Brown & Ryan, 2003). Mindlessness is a state of existing without awareness of what is happening in the world around you and within yourself. Mindlessness can take many forms: letting your mind wander (Killingsworth & Gilbert, 2010), running on autopilot (Bargh & Chartrand, 1999), and even suppressing unwanted experiences (Kang et al., 2013). In this last case, mindlessness may be defensively motivated - you may refuse to acknowledge or even actively suppress some thoughts, emotions, motives, or sensations if they are uncomfortable, embarrassing, or otherwise unpleasant.
Mindlessness is often the default state of being - it can be very easy to drift into autopilot, to let your mind wander, or to suppress unpleasant thoughts. However, this state of existence often leads to longer-term unhappiness - mindlessness can lead to anxiety and depression as well as impaired creativity and problem-solving skills (Kang et al., 2013). Although mindfulness may be more difficult and less comfortable than mindlessness, at least at first, it often leads to better functioning in the longer term.