Pokemon Trainer takes a while to get. It sucks the first couple of months because you will constantly lose to opponents worse then you. The trick is fully understanding the pokemon and mastering the switch dynamic. I do wonder why you aren't able to kill with Ivy though. Are you properly utiliizing uair? Also learning to spike with dair is essential. Don't be afriad to go out for the kill with dair. You'll suck at it at first but eventually once you get it, things will start clicking.
For me when I started playing PT, I had a decent squirtle and charizard was my best, but once I had a better grasp of Ivysaur's strengths, wins starting getting easier. It's not too bad to use Charizard for those K. O cleanups, but flare blitz should really be utilized as a roll punisher. Up throw, fair, and up smash are much safer options on the zard.
In regards to King Dedede, first respect the D. He's not optimal but he punishes mistakes hard (which as a new PT you will likely do often). Squirtle turns him into combo food and ivy has a strong ledge game/distance game against him. In this matchup it shouldn't be too hard to kill him with a strong upair or upB from Ivysaur. Regarding Charizard, he should only really come out around 120% on your opponents end for the easy kill throw (if you are struggling killing with Ivy) or when you are a high percentage to make use of his weight and rage.
