FG% is pretty basic, I used it only because it's quick and easy to calculate. You can toss in scoring efficiency numbers that include FT%, it won't change the story all that much (because he isn't as good a 3 point shooters as most other high volume wings). His TS% against Boston in that series was a pretty mediocre .519, especially compared with Tatum (.588) and how he played earlier in the playoffs against Milwaukee (.671). Butler was the reincarnation of Jordan against the Bucks, but against the Celtics he was far more ordinary. Butler's late game efficiency was a huge part of why they won 1 and 2 (and almost stole 6), but ultimately Miami beat Boston because their 3 point shooters shot out of their minds on wide open 3s, and Boston underperformed in that regard.
In the regular season, the Heat (outside of Butler) shot .343 from 3. Against Boston and excluding Butler, they shot .445 from 3. They shot more than 100 points better from 3, with the only real personnel difference being that they were missing Herro, who had been by far their best 3 point shooter during the regular season. That's mind boggling, and doesn't have much to do with Butler (especially because we were rarely doubling him). Miami shot an insane 58% on wide open 3 pointers during that series, the best of any series (by several points) in the last decade. Jimmy was far less efficient a scorer in that series than he had been for the entire regular season (.647 TS% in the regular season), yet the guys around him performed far better than in the regular season. Why?
Meanwhile, Boston shot over 70 points worse from 3 (.377 down to .303) in that series compared to their regular season numbers. It's infuriating to say that sometimes it's just a make or miss league, but...sometimes it is. Butler wasn't this unstoppable force against Boston, and he wasn't the biggest reason why they lost.