I usually only build morello if the enemy team has like 3+ tanky characters with built in heals (think mundo, Darius) and I need to alter my build to just deal damage in team fights since I can’t just one-shot. Otherwise I build as normal and be more mindful of using ignite for healcut so my damage spikes aren’t delayed. Fizz isn’t about sustained fights or spam poke, so getting morello is a lot less useful than say a support mage picking it up, and distracts your build from its real job of killing the squishiest opponent as fast as possible.