From SmashWiki, the Super Smash Bros. wiki
The Score Display is an unlockable optional rule that appears in Super Smash Bros. Melee, Super Smash Bros. Brawl, and Super Smash Bros. 4. It is unlocked during Versus Mode by scoring a total of 5,000 KO's in Melee and a total of 200 KOs in Brawl. While this rule is switched on, the net score of all combatants in Time and Bonus matches will be displayed on-screen next to their series symbol. In Bonus mode, it displays the same score as if it was Time, and not the amount of Bonus points collected. The Score Display does not appear in Stock and Coin matches.