Hey bro! the best thing that you can do against Cloud as any character is to do whatever you can to edge-guard him. When you play against cloud, have that as your win condition in mind. His recovery is absolutely one of the worst in game. If you can get him off stage and hit him with anything he will most likely just die.
Specifically as Ness, you have some pretty nice tools to make this happen. Ness has some great aerials to edge-guard Cloud, as well as good throws to get him off stage. Additionally, grabbing Cloud in general is really good since his OoS up-B is one of his best tools, and will often be shielding to try to get it. Ness also has great specials to keep Cloud from coming back to ledge. PK fire and PK thunder are great options that put you at pretty minimal risk. I've gotten gimped by PK thunder as Cloud more times than I'd like to admit.
The bottom line is that Cloud is gonna try to space you out with Bair and Fair, get you with Up-B Oos or U-smash your jumps, because thats just what he does. Your goal is to play around these as best as possible, keep your movements ambiguous and the moment you get a chance to get him off stage, CAPITALIZE.
Best of luck my friend, hit me up if you wanna practice!