Do the letters from “E-S” really mean anything for the monsters?
If so is it worth it to farm for better letters?
Those are grades
The higher the grade, the better chance you have at getting a bonus action, which is another attack, basically
Do it if you’re a completion freak
If you just start playing this game, don’t bother with grades. Arks before end-game are not worth the time to farm an S grade. Enjoy the game, and you will do fine without any bonus actions.
Except for a few. You may have to go back if you want to farm for good S grades. But as First Snow says, you’ll do fine without any bonus actions.
Yeah the game is pretty easy to defeat
Grinding for Arkadions with a high grade can be done at nearly any time, but I always find it better to save that for end game. Why? I don’t like to frustrate myself with the possible long hours of grinding for one arkadion until I can experience what the game has to offer 
Thank you for all the responses it helped a lot!
No problem , glad to help!
F, E, D, C, B, A And S for grades S makes the card shine