She's still the Soldier 76 of heals. Super consistent output, mobile to boot. Even on your worst game as mercy, the heals will flow, the rezzes will happen, and you'll drip value.
The difference is, Mercy is only competing with 4 other heroes for the support slot, not 14.
tl;dr: She's still the most reliable healer in the game, and Valk is still strong. Also, Rez is still powerful and here to play. It's just not free during Valk anymore.