Why were the Knights Templars so useless?
Glossing over the fact the crusaders were never involved with the Mongols in this fashion. Mongol armies were most effective as mobile hit and run troops in open areas. The Crusaders were most effective as either heavy shock cavalry or close quarters specialists. Therefore I think it's fair to say a heavily armored crusader would annihilate a Mongol on the ground just like a Mongol would likely annihilate a crusader on a horse in the open.
I mean common the Knights Templar history is replete with examples of outstanding fortitude and courage against overwhelming odds. However, the knights portrayed in the show seemed to die like they weren't even wearing armour, none of them seemed to have shields and most seemed to lack any of the formidable marshal prowess they possessed in real history.