Ivysaur would be so ridiculous if it were not weak offstage with the recovery change being significant anyway (it's weak out there now, but that's a bit improvement from the previous "horrible"). If Ivysaur gets joined off-stage by a competent edgeguarder like Meta Knight, Ivysaur is in trouble. If Ivysaur can make its moves to safety before they have physical time to get off-stage to it or they try to edgeguard Ivysaur from the safety of the stage, Ivysaur isn't in such a bad position. Ivysaur is a little character with big range and big damage potential, and history shows us those characters are inclined to work well. Ivysaur is also slightly heavy instead of very light like the other cases. All in all, Ivysaur seems to do his job in the team pretty well to me and is definitely extremely unique.
First, I will quickly address the statement that Ivysaur is a little character with big range and big damage potential, which is somehow always a winning combination.
Firstoff, Ivysaur is not small, so that part of the statement can be readily discounted. The other characters who you probably are trying to mention are Metaknight and G&W, who are also misconceived as small characters when they actually have rather wide rounded frames overall. G&W however is a small character when he's crouching though. Want a better example of a character who is small but not a lightweight? Mario.
Second, as for big range and big damage potential, let's just say Ike fails. Poor mobility, limited safe strategies, poor setups into kills, and a limited offstage and followup game makes him a bad character in vBrawl (while he's still unimpressive in BBrawl).
So how is it that Metaknight and G&W are successful characters (not to mention Marth)? Key thing is these characters move a lot faster, and have MUCH less ending lag on moves that matter, whether it is kill moves, poke options, or pressure tools. Furthermore, these characters have good followups, which when compounded on massive damage potential, becomes ridiculous.
I should add that Ivysaur isn't necessarily always heavier, as a well-placed fire attack can put a quick end to her.
So basically Ivysaur as I was saying is like vBrawl Ike with the failures of a tether grab, but not terrible at spacing, and with a mediocre projectile to randomly throw out. All in all pretty bad, but not bottom tier bad. You can annoy your opponents a bit with B-airs and Bullet Seed and throw out Razor Leaf at mid-long range, and you have little else that stands out besides a few gimmicky kill moves.
If Ivysaur's recovery was somehow extremely good while keeping her onstage game exactly the same, she would probably be mid tier. Decent overall, but not overpowering. The only characters who can claim to get away with a bad recovery and be good in this game are Olimar, and Link imo. Link is inferior to Olimar due to his grab being worse however, but he sorta makes up for that with the benefits that come with a Z-air and some very versatile projectiles, including ones that are very good for gimping.
Lastly, I'd like to know why Ganon's Jab only does 9 damage maximum, which is exactly the same as Ivysaur's B-air, and yet it is the only Jab in the game that gets punished on spotdodge, not to mention it is too slow to be viable as a spotdodge option.