Look at the bat rack in any MLB dugout and you'll see a colorful array of numbers, thanks to the trend of bat-knob stickers taking off in recent years. Most players stick to their jersey numbers, but Bryce Harper of the Washington Nationals isn't most players.
The reigning NL MVP keeping it 100 -- he's one home run shy of that milestone number -- is just the latest salvo in his "Make Baseball Fun Again" campaign. He told ESPN the Magazine before the season, "Baseball's tired. It's a tired sport, because you can't express yourself. You can't do what people in other sports do."
Using an emoji as a bat-knob sticker certainly isn't tired, and it certainly qualifies as Harper expressing himself, though it remains to be seen if he'll actually get to use them in a game, since MLB reportedly banned personalized stickers before the season. But as long as he's putting up numbers like he has to start 2016 (two homers in five games, 1.556 OPS), the Nats probably will be fine with Harper expressing himself like this.