Kirby is the strongest. Yes, the small, pink, spherical vacuum is - in a way - stronger than every other character on the Smash Bros. roster.
Kirby was first introduced to gamers in 1992, when Kirby's Dream Land was released for the Nintendo Game Boy. Kirby was created by none other than Sakurai himself. Although Sakurai hasn't played a part in developing a Kirby game since 2004, some of the greatest Kirby titles were produced under his direction, including Kirby: Nightmare in Dream Land and Kirby Air Ride.
Kirby has always had an unassuming demeanor, but he actually holds great power. After consuming his enemies, Kirby can transform, gaining some of their abilities. This has been a major gameplay mechanic in almost every Kirby game, and it has been a defining feature for his character in Super Smash Bros. It's also what makes him the strongest character on the game's roster.
How Kirby Is Stronger Than Every Other Smash Bros. Character
Players already know Kirby is the most resilient Super Smash Bros. Ultimate character, from an in-universe perspective, as he was the only one to survive the World of Light's opening cutscene; however, his copy ability also makes him the strongest. During Pyra and Mythra's Smash Bros. character overview, Sakurai revealed that all of Kirby's copy abilities are stronger than the original moves. He then demonstrated this by having Kirby and Mythra use Lightning Buster on one another, showing Kirby dealt more damage.
This damage amplification, applied to every character movie Kirby can copy, proves Kirby is the strongest character in Super Smash Bros. Ultimate. The saying, "Anything you can do, I can do better," applies perfectly to Kirby's transformation ability, and being able to master each character's abilities in a matter of seconds makes him an even bigger threat. While there is a size and weight limit to what Kirby can inhale, some gods and goddesses are plenty small for him to such up. If Kirby were to swallow someone like Kratos, a demigod who has killed multiple gods, he would not only be able to copy his abilities but would be even better at them than Kratos. Of course, Kirby is more concerned with his stomach than becoming the most powerful being in the universe, which is why he doesn't seem like a major threat. However, that doesn't change the fact Kirby's version of Roy's Flare Blade is the strongest neural special in the game.