Uh, we have also had buzzer beaters. Every team has those.

There have also been games that LA has lost when Kobe failed on buzzer beaters. This is a questionable line of reasoning.

The issue the Celtics isn't actually the record. It is the team's consistently poor performance this year. We had the best record in the NBA for many weeks. Then we started looking horrible for 2 months.

A win is a win.  If you win on a buzzer beater in the playoffs, it's a win.  No different than the regular season.

On the other hand, the fact that the Lakers have struggled to beat lesser teams on numerous occasions does tarnish their aura of greatness a bit.  That doesn't come close to the Celtics blowing so many double digit leads over the course of the season, though. 

The bottom line is the Lakers can rely on Kobe to win them the game more often than not when they get themselves in a tight spot.  Unless Ray or Paul is having a hot shooting night, the Celtics don't really have somebody who can bail them out.