Yeah, but you should probably mention that Yennefer had been seriously wounded just before that, and was lying on the ground bleeding with the mob approaching. She wasn't capable of defending herself at that moment. So, she asks Triss to teleport them. But, of course, Triss stands her ground and together they perform a spell that disperses the crowd.
Yennefer is not a coward because of that. And it is only Yen calling Triss out, that made Triss show bravery and help her.
And if you were referring to that scene you shouldn't have claimed that Yen was "frightened and want to teleport herself far away without thinking about Geralt". Because as I showed, it was the other way around.