Ivysaur doesn't rely on her ground game at all. Her strongest points are in the air.
I suppose I didn't explain my point well and pulled the definition out of nowhere, sorry about that. I was trying to say something else: you would say its (sorry, the "her" sounds weird to me, My Ivyz00r has a very manly pokemanz voice
) best moves are its aerials, but what I meant was: Ivysaur is not exactly an aerial fighter (Squirtle is, he has the air speed and quick aerials that allow him to be a better aerial fighter). In my experience, to keep up with Diddy, I found you needed those two (air speed and quick aerials) more than long-ranged aerials; also, Ivy's more bound to the ground for obvious reasons. Now, bair is a great move to have, range-wise and as a set up, but doesn't make up for the intense damage-racking abilities of Diddy. I don't think you can keep Diddy away the whole match dealing 2-4% damage per hit. Nair, on the other hand, is probably more effective as an offensive approach but more risky.
I think Ivy's best bet is probably trying to get control of/get away bananas and using bair's range to avoid falling into Diddy's traps. I'm curious to know what the Diddys think about this match-up though, it's the one I need to know the most. I also think Razor Leaf is quite important for Ivysaur's defensive purposes here, but I might be playing it wrong.
God I was probably on crack about the ground game thing, it's more a complex of seeing Ivysaur on four legs or something but I myself play the aerials more than the rest obviously. If I got anything else wrong about the match-up though, please tell me, I'm here to learn after all so corrections are appreciated.
Diddy's Air game is equal to Squirtle's air game, so you can't say Squirtle is safe in keeping everything in the air as Diddy will adpat to that as well. Squirtle can put on a few things on Diddy but for the most part the two pretty square even almost.
As Steeler said, Squirtle's advantage in the air against Diddy is air speed: Squirtle will probably outspeed Diddy in the air more than the opposite. Also, never mentioned safety, nothing is really totally safe. It IS the safest way to face Diddy though.