Smiling because Daniel Carrico greeted him. He wasn't laughing at being relegated

what did you want him to do start crying?
The thing is that smiling isn't the whole issue. Bosingwa, and an awful lot of the QPR team, haven't looked bothered all season. He did absolutely nothing yesterday, and that epitomised how he has played for them.
When he walked off the pitch grinning at the end, it did not look good. A number of the rest of the players at least stayed on the pitch to thank the fans, and he didn't even do that.
I've never been somebody who's believed much in loyalty to clubs - ultimately, being a footballer is a job, and players should do what is best for their career. I'll never vilify somebody like RvP for leaving Arsenal for Man Utd, as realistically it was his best chance of winning the title. As a Liverpool fan, I would not have had a problem (morally) with Steven Gerrard leaving for Chelsea a few years back had he done so as, again, it would have given him a good shot at the title.
However, as with any job, footballers should put in 100%. That is what they are being paid to do, and it's only right that they do this. You can forgive players an off day, but certain QPR players, and particularly Bosingwa, have offered nothing like their best this season, are the reason for the side going down, and then have the audacity to leave the pitch chatting away to their mates on the other team.