That is a lot more common than fans remember. The internet age and the fact that Jordan was really only in the Finals in his prime (and thus didn't have very many stinkers) have made it seem like it is rare for a star to not play well on the biggest stage and that just isn't reality.
That was Bird's first finals and his point totals were
18, 19, 8, 8, 12, 27
Imagine how much Giannis would be getting killed if he did that in the Finals.
Also for a comparison, Lebron against the Mavericks
24, 20, 17, 8, 17, 21
Obviously that was not Lebron's 1st time in the Finals and the Heat lost as opposed to winning like the C's, but Lebron outperformed Bird, yet Lebron is often thought of as the worst Finals ever from a great player, while Bird is never mentioned. This media era is just so much different.