Your performance makes a difference. You need min C to get decent loot, but better with B. You also might want to hunt big enough titans to get better chances for ascension materials.
Beware of confirmation bias. Our human minds have difficulties judging probabilities. You need to gather data to evaluate. For example only 1 person gets A+ loot in an alliance, that is like tossing the dice once. But 20+ people get B, that is like tossing the dice 20 times. No wonder people with B report more “6” (good ascension materials), because 20 tosses have higher chance of something good than 1 toss. But it doesn’t mean B tier gives better loot or have higher chance.