Snake has a lot more options in the air than he did back in Brawl, so this matchup got a little trickier than it used to be. However, the idea is the same. You don't want him to set up all his gadgets and toys. Snake, if left to his own devices, will lay out explosives everywhere and force you to approach. This is how he wins the neutral. Luckily, Squirtle has a great tool to prevent this; Water Gun. Water Gun, if not blocked, will immediately push the Snake outside of his comfort zone onto the ledge or (if you hit him really well) off the stage altogether. Once you're in this position, his options become way more limited. Water gun him whenever he starts pulling out grenades or dropping C4 and you no longer have to worry about it. Snake will probably start shielding the water gun if you span it too much so it's important to wait for him to be busy pulling out grenades so he won't be able to shield. Aside from that, you just need to be patient. Up-tilt will kill you embarrassingly early, so don't ever attack his shield. Water gun and patiently wait for an opening.
Ivysaur can play the camping game with Snake, and can be very annoying for him. Razorleaf can blow up grenades in his hand, or stop grenades that are in the air. If played well, he'll have to approach. This is where Ivysaur's midrange zoning really shines. Razor leaf is your bread and butter here, with f-air and b-air as really good offensive and defensive options. Also, look for up-b opportunities.
I tend to sit Zard out of this matchup. His big body makes him too easy to hit with all of Snakes explosives, which makes it really hard for Zard to approach. I'll normally just switch back to Squirtle to win the neutral with a quick water gun.