Ghost battle will make you worse. It's good for killing time and getting unlockable outfits. Maybe it has a use for practicing combo's in a match, but in the end, I feel that you learn more about how to abuse the AI in ghost battle mode than actually learn Tekken. When you fight a legitimate human being, the game flows drastically differently.
Go into practice mode, learn some core moves and simple juggles to get the hang of the game. Then go online and test what you know.
If you lose, go into the replay data and watch the match you lost in. Notice what moves your opponent used on you to win.
Go back into practice mode and learn how to beat that. Boom, your knowledge increased and you've levelled up. Rinse and repeat until you become a Tekken God
Your execution will automatically improve with as many games you play, but the key thing is to always find out what you're doing wrong and fix it. That's the key. Don't let the bad habits consume you.