This Match up is either even or maybe a very small advantage for Yoshi.
I also play Ness so I'll try to explain this MU in both sides:
Yoshi's strong points:
- Yoshi has better tools in neutral than Ness and wins it most of the time (most characters do anyway)
- Yoshi abuses' Ness disadvantaged state very well, specially if Ness is above him (as @
GSM_Dren already stated)
- Yoshi controls the stage very well against Ness thanks to his better mobility.
Ness' strong points:
- Has better reward than Yoshi overall, specially with his combos at low %s.
- Has good kill moves and they have a easier time hitting than Yoshi's does.
- Can give some trouble with PK Thunder, but not as much as he does against other character because of Yoshi's aerial speed.
So basically, Yoshi dominates Ness most of the time, but Ness kind of balances it out by being more rewarded than Yoshi when he gets a hit and being able to land kill moves with more ease.
As for the stages, I personally like stages where the Neutral game is more of a factor, such as Final Destination, Smashville or Lylat Cruise.
Battlefield and Halberd benefits Ness more than Yoshi so I don't recommend those stages.