Oh hi. You guys may not know me, (ok Z'z and Tarex know me lol) but I feel the Ness MU is +1 YOSHI'S favor. I played Tarex's Yoshi but it was my 1st tourney so take what I say with a grain of salt. =P
One thing I really loved Tarex did against me (I think he does this against several others, though I'm not too sure XD) is that he used Egg Toss a lot and I got too focused on shielding them that I forgot about Egg Lay and grabbing.
Anyways...
Ness's PKF isn't really useful against Yoshi as Yoshi can easily escape it with his DJ Armor. Yoshi will only take like 5% at the most.
Ness's PKT most likely won't hit Yoshi from far, as Yoshi can Egg Toss Ness.
Yoshi can GR>U-Smash/U-Air Ness, which is bad for us especially at high percents :/
Try not overuse Yoshi Bomb as Ness can heal from the stars that come from it. However, he probably won't get a chance to heal from the stars if it is lag cancelled on a slant.
Yoshi pretty much outranges and outspeeds most of Ness's attacks, another bad thing for us.
However... when Yoshi is off-stage that's when I feel Ness can shine. He can attempt a Dair spike or bait an airdodge and try to go for a Bair or Uair. Yoshi's options when he's offstage are pretty limited.
Ness can kill more reliably imo, he has Bair, B-Throw, Nair (not as great as a killer but still decent), Dair (as a spike
), Uair and PKT2 (though PKT2 is pretty unlikely and trying to tailwhip then PKT2 wouldn't work too well as Yoshi DJ armor goes through tailwhip) while Yoshi has Uair and some others options that I don't know too well of. =P
In the end, Ness has to stay mid-range. Far away and Yoshi can Egg Toss him like there's no tomorrow and Ness can't camp Yoshi, while close range Yoshi outspeeds and outranges Ness. And Yoshi can grab at close range
Again, this is just my opinion and I don't have THAT much experience, but hopefully this helped. And I'll get better at writing up MU summaries with more experience XD