for my experience for yoshi specifically
yoshi's combos are really good...but only mostly vs the fast fallers or heavies, yoshi does bad vs grabs and most projecitles (as in fox's lasers/throws, and falcon's throws)
1. Pikachu: pikachu counters yoshi (60 40), yoshi doesn't have as many combos vs any of the lighter characters, pikachu has a recovery that is difficult to gimp, plus....pika's back throw
2. Kirby: Kirby definitley (80-20), I say yoshi's worst matchup, same deal as pika...but even less combos and difficult to counter any of kirby's combos, yoshi has problems countering any muti hit down aerial
3. Ness: (45-55) Even or slightly in yoshi's favor, the problems are from ness's djc's and throws, but yoshi has many combos and can easily gimp ness's recovery...also, yoshi can get out of many combos through djc'ing
4. Fox: Fox (70-30) due to lasers and throws...its very difficult to get out of fox's combos, lasers make approaching hard, and tap a combos by fox are incredibly comboable, yoshi has djc combos including up air/up tilt and n-air
5. Captain Falcon: Falcon (barely...like 55-45) due to throw combos, yoshi has a problem djc'ing against many of falcon's combos....but yoshi has many up tilt and up air combos, falcon can 0 to death, but yoshi can also)
6. Jigglypuff: Another bad matchup, 70-30....yoshi has very small amount of combos....can't gimp ko through up tilt.....Rests and up throw combos can kill yoshi easily
7. Mario: even, problem being throws and fireballs...but yoshi can combo well vs mario,
8. Yoshi: uhhh wut
9. Donkey Kong: problem is throws...but dk is a lot slower than falcon, yoshi has 0 to deaths, dk has mindgames...take you pick, i say yoshi 60-40
10. Luigi: Luigi has combos, yoshi has some even though luigi is incredibly floaty, but up b really kills yoshi and yoshi doesn't have that many gimping moves, 60-40 luigi
11. Samus: Yoshi 60-40, yoshi has combos vs samus, samus has mindgames, samus has a bad laggy throw so that doesn't help her
12. Link: Yoshi (55-45) link's projectiles really hamper yoshi, Yoshi has a huge amount of combos...link is easily gimped