Ice Blocks are classified as Living projectiles, like Olimar's Pikmin. This should mean that if Ness's PK Missile connects with an Ice Block, the distance traveled is significantly decreased. If Ness is recovering from below the stage, you can even shoot Ice Blocks continuously and force him slowly down. His second jump, even when covered by f-air, is stopped by our b-air. In most cases, we trade hits with it. Ness's recovery fails very very hard against the Ice Climbers.
Beware of his PSI Magnet. Blizzard is considered an energy-based attack, and Ness can absorb it for free healing. Significant healing, considering how much damage Blizzard can do... Blizzard should only be used to intercept Ness's approach, and even then with caution.
I'll write more stuff later, I need to go cook dinner right now.
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Alright, more stuff. Squall's pretty good if you use it from up-close, but using it as an approach is quite punishable by PK Fire - it's a Laser projectile, meaning it goes right through your attack. And we all know what happens when Ness gets a PK Fire in... Worst thing is, he can b-throw Popo while Nana is still tormented by the flames. Ness's b-throw is strong.
Recovery is a bit risky due to PK Thunder, but only if you give Ness time to make it risky. Recover fast - PK Thunder is faster than you'd expect. Squall or Belay can be safer depending on the situation, but usually Squall is the better option. When recovering from below the stage with Squall, watch out for PK Fire. It'll be angled down, meaning in your direction. On-stage, PK Thunder won't see a lot of use since Ice Blocks easily intercept a controlling Ness. Still, be aware that it's there.
Ness's air game is pretty, but beside b-air, there's not much you should be particularily cautious of as long as you're on-stage. F-air and n-air can be shieldgrabbed, d-air is easily prevented with one of your upward-aimed moves. B-air has a good bit of shieldslide when sweetspotted, though. Still, it's telegraphed a fair bit, so a spotdodge to grab will solve it.
Ness's yo-yo is a pain. His charge actually does damage (meaning it's nigh impossible to powershield), and the moves that use it aren't very laggy, while both of them hit on both sides. The d-smash also has a pretty good range. If you see Ness charging his yo-yo, stay out of range and use Squall once the hitbox goes away.
F-smash has loads of start-up lag, and won't be used on a grounded opponent. When recovering, make absolutely sure you land your Squall on-stage. Going into helpless will provoke a f-smash, and it will connect. Ness can also use it during PK Fire, so smash DI away from the pillar ASAP.
His tilts are fine, but we outrange them with our own tilts. Sadly, Ness's f-tilt and u-tilt have godly priority. F-tilt is our best counter, being more ranged, more disjointed and about as fast as Ness's tilts. It'll either hit Ness or clash with his tilts. Both are fine - they just reset the status quo, really. His d-tilt trips at a very high rate, so don't think Nana can save you from his f-smash when you trip. Fortunately, the range on this move is abysmal.
I'd say the match-up is very dependant on the players' positioning. Long-range is in our favour, mid-range is in his favour, close-range is in our favour, melee is once again his favour. Both characters can do a lot against eachother's recovery, too. This match-up may be among the most even ones we have... Though, my experience against Ness consists of only one Smasher, so maybe my advice isn't always valid.
Anyway, if it wasn't obvious from the last paragraph, I'd put the ratio at 50:50.