Quirks effect gameplay ?

SDS

Or are they just there for “show” based on attributes. Asking about all cards not just lice series

SDS

Live series cards have what’s known as active quirks. These are situational quirks like first pitch hitter, dead read, unfazed and so on. Basically you will receive a boost in contact (bigger pci) in the given situation. Legends and flashbacks can only have passive quirks, like bomber, speedster etc. These are solely based on stats and have no extra effect in gameplay. A card with 75 power both sides will always have the bomber quirk, but receive no in game boosts.
There is a list of quirks and effects at the link below. It’s from ‘17 but they’ve not changed in the interim to my knowledge.

http://www.sportsgamersonline.com/mlb-show-17-player-quirks-guide/