Yes the God-ending is only available if you play as Avatar-Karlach. And I would argue that it really isn't a good ending for, especially if you look at what a blast she and human Gale seem to be having in Avernus while fixing her heart.
Of course interpretations can vary but Karlach's main issue was that she didn't want to be alone (without any nice friends) in a horrible place, having to do Zariel's bidding. In her Avernus ending she is not alone, she has the player-character (her love-insterest if you play as her) and / or Wyll at her side, and a remedy for her heart in sight.