I know this is just some other noob talking, but I believe it's how much passion you have for making a character affective. Keep in mind that any character can punish and be punished. Ivysaur's upsmash... is... well..
*Jesus sneezed on Ivysaur and give her the ability to kill at 0% *
That move is totaly rediculous. When it comes to a bad matchup with Ivysaur, that's when your knowledge of the other character comes into play. Not only that, but their playing style. Punish whenever you can because, not only the upsmash, but most of her moves can harshly punish players to either run into more dmg, or a misplaced stock.
My friend has a bunch of hacks for the Wii so I get to practice with all the pokemon individualy. Ivysaur is something I'm trying to work with more now( next to Squirtle).
So yeah, basically, NEVER GIVE UP ON YOUR PASSION AS AN IVYSAUR PLAYER.
YOU GOTTA CATCH 'EM ALL!