No its not.

The "I PK at towers so I'm legit and you're a booster for mob PK'ing" thing is a little flawed because towers are also mobs of players where you just spam your sword and hope you get the kill, might be less people than at huge mobs but it's still the same thing.

It's just a way for people who PK nonstop to make them selves sound better

I mean people can boost as Bryan mentioned above, but 99% of the time someone calls someone a PK booster the person isn't actually boosting.