Poker is a skill based game, with elements of luck involved.
This means that, just like in any game, profession or sports, you need both theoretical and practical knowledge to be successful. Just like you need both hands to make a clap, in poker, you need both studying and playing real hands in order to improve.
Doing just one will not do you any good. So, my answer is yes, you definitely need to study poker to win. But you can't invest all the time to it of course. You should find the perfect balance between studying, playing, and resting as well.
I would suggest 5 hours of studying and 20 hours of playing per week, if you are taking the game seriously. If you have more spare time than you can increase the hours correspondingly