I haven't read his comics, but I have watched his cartoon. Does Maxx have powers? I don't see him perform any superhuman feat.

That's pretty much the point.  He isn't really a super hero in the conventional sense.  He was just a homeless guy who crashed into Julie's world, and became so wrapped up in it all that he stopped being 'Dave' and believed he really was a superhero.  
 
I think in Julie's 'Outback' that he was super strong & athletic.  However, in the real world he was a nut-job who wore bright purple & lived in a cardboard box.  So other than his big muscles, he is just the poor homeless guy who Julie accidentally hit with her car.       

It kind of works that he has superpowers but only in Julies other reality, he can kick ass though, he is just a big strong guy, think Marv from Sin City

He did dip his hand in lava in the outback, you have to understand that the Outback is a real place that Julie created. The lampshade that covered  his body after Julie hit him with her car became his mask. It's alive and crawls on his face. It becomes part of him, he is really irrelevant without Julie he would be nothing just some guy named Dave. Plumber, gardener, it doesn't matter about Dave before Julie. The power she has(had?) was enough to bring power from the outback. Take the Isz for example, they came over but turned black and a bit twisted. Dave went to the outback and came back just as changed, although he's stronger in the outback. But his powers in the real world are limited to having some lava claws and teeth that sometimes bite other people.  In the Outback he's a big bunny rabbit with claws.