Link being -1 against G&W? I'd like to think that it's because Game & Watch has overall better priority and kill power, really. I mean, G&W's aerial game is really what makes the MU difficult, but Link can still win (hence why it's only -1 and not worse). Link has great disjoints on the ground and solid projectiles that can keep G&W at bay, and it's not like Game & Watch can Bucket Link's projectiles either, so Link's long-range options are safe. Then there's the killing; Link usually has a hard time killing in general, but thanks to the light weight of G&W, Link can get a kill on him fairly easy, though one has to keep in mind that G&W can Bucket Break. Really, the only real problem for Link in the MU is the fact that Link can be easily gimped, and G&W's Fair can do that easily. Maybe that's what makes the MU a -1; the fact that G&W can gimp Link so easily.
As for the Tink vs. Samus MU, I couldn't really tell you.