Before her rework? Before Immortality (after Resurrecting)? Nope. She was almost never used in pro play and never saw herself in games above diamond. The only reason her pickrate was high back then was because her skill floor was relatively low so not-so-good players - mostly gold and plat, played her. These players make up a large portion of the player base. Additionally, even after the buff that made her temporarily invulnerable (which by the way was added because devs said her ultimate was pretty bad as Mercy almost always died right afterwards) nothing much changed. Other than the bs “Hide n Res” strategy that people started to use because temp immortality literally incentivises it (even though it’s a pretty rubbish strat).