s almost every game

It compares you to other Taliyahs in your role. A lot of times you can just take a champ to an off meta role and as long as you do somewhat decent (positive k/d, high participation/farm etc.) you'll get an S because the few other people that played that champ in that role probably fed. Ex: Played Gnar support; score was nothing special but got an S because I did far better than the average Gnar support.

Because it is based on the average of all people playing the champion.. and everyone else is bad at Taliyah

The majority of people that are playing Taliyah have no idea what they're doing.

The standards are very low, due to Taliyah's overall poor global performance.