Alongside the Rules Lawyer, this is one of the most disliked player archetypes on /tg/, as the general perception of them is the Chaotic Stupid player. Unlike a Powergamer or Leeroy Jenkins, a Loonie doesn't care so much about "beating" the game or even combat at all; they just want to have fun. This often translates into both "wacky" character concepts and outbursts of random/stupid behaviour if things start moving too slowly for them, 'just to make things happen'. They often have a very short attention span and whilst they don't necessarily want to be the center of attention, they do want stuff to keep on moving.
To put things in perspective, the Kender and the Xaositects were basically tailor made to appeal to the Loonie's playstyle. Which brings up the question of whether a Loonie who is really playing to the Chaotic Stupid aspects of these guys is still the Loonie or has gravitated to being a Real Roleplayer. Most fa/tg/uys don't care, and just punch them in the face when they inevitably push their limits.