He is a winner because he has won. The other ways of defining "winner" come down to a popularity contest. This is the attack that anti-pierce posters used on Pierce before he won a championship.
That being said I think Kobe as a player is overrated. His talent is overstated and his hardwork is underrated. He takes good care of his body and plays extremely hard in the regular season.
But in this league he just IS NOT the transcendent talent he is made out to be. Back in the day it was Jordan>>>>>>>>>>Glen Rice (or whatever two guard you want to insert) and everyone else.
Nowadays there are LOTS of guys who can hang with Kobe. Guys like Dwayne Wade (yes I said it he is back), Iggy, Pierce, Lebron, and more. Kobe might be better then those guys for the most part. But the gap isn't anything special. Depending on the matchup he doesn't have any advantage at all! It's not really Kobe's fault. It's the rest of the league has caught up with Jordan. Jordan was the best package of athleticism/basketball knowledge/competitive play we have ever seen. Kobe is remarkably close - but so are a ton of other guys.
In fact guys like Pierce can rise up and outplay Kobe in a critical series. That was unthinkable in the day. Talk what you want about supporting casts but there was zero chance of any 2 guard/forward just coming in and owning Jordan for a series.
That's the real reason people like to bash Kobe and call him not a winner. It's simply that his competition is up. Think back and think about the other top "two guards" in Jordan's day.. The list is remarkably small of dynamic players like Kobe. Jordan has really changed the game. It's the media and some in my view misguided fans like to harp on him as the best player in basketball. Even if that's true it's not terribly significant anymore. The Celtics proved this first hand. He just can't exert his will like Jordan could. Jordan would have shut down the Pierce equivalent while scoring an outrageous amount of points...
With Kobe greatness was thrust upon him. He never took the mantle of greatness for himself. In fairness to him he should never have been expected to be the next Jordan. Expecting a guy to come in and be a basketball god and having him seem quite human is where this "not a winner" stuff comes from. The truth he is a winner but is undeserving of basketball god status. Yes the 81 point game was amazing. But Tony Delk had a 50 point game... It was something of a fluke.