Okay, so I just did some testing on Brinstar, and if this isn't already listed as "one of kirby's best stages", then I think maybe it should be. Here are some reasons it's so good for him:
-Close sides for early KOs from fsmash, dsmash, hammer, aerial hammer, wall of pain, some copied powers.
-Short ceiling for early KOs from upsmash, dsmash, aerial hammer, hammer, upthrow, upair at high %
-Spiking people into the acid
-I figured out what happened on the breakable cylinders that can separate the stage. If you run onto them, and do a grounded hammer, you will actually slide a noticable distance, and your hammer reaches much farther than it normally would. TRY IT!
-Increased hitbox duration for Stone and downward-angled hammer on those breakable cylinders.
-Jab the string pillars to refresh moves.
-Leftward-facing Final Cutter on the right platform (attached) follows the platforms angle downwards and can hit people on the middle part of the stage.
-Stone Shockwave Glitch on the right platform (when string is broken). Beware of falling off when the string reattaches, or when the stage comes back together.
-Turning into a Stone when the acid is high for safety. Your opponent can't follow you there. You will eventually take over 30% damage from the acid in Stone form, and it will knock you up, but you won't take any damage due to the Stone's mechanics. Beware of doing this at high percentages.
-Attacking through the bottom of the stage is great for Kirby. Beware of characters with spikes.
-The small stage discourages camping. Campy characters are screwed even more when the acid rises to full height, and you are both pretty much forced to the top platform.
-Kirby evades the acid easier than most with his multiple jumps.
Things to watch out for:
-Falling through the acid as a Stone to your doom.
-Close sides and top can be advantageous for your opponent, as well as you.
-Some attacks can last incredibly long on the strings that attach the platforms. Take advantage of people that do this by accident, but watch out for people who try to trick you with it. (Snake's Fsmash, like Bunny said)