One thing you should avoid doing is rolling, Shiri plays me all the time and that is where the majority of my kills come from is punishing yoshi's rolls. The timing isn't that hard to figure it out, and samus' fsmash is perfect for punishing rolls. I also use the uptilt at lower percentages, as it is a nice set up as well, avoid samus uptilt at all costs, it combos into anything and will steal your second jump at much lower % than you think. Another thing you should avoid is staying in your shield up close to samus. Her jab/ftilt combos are more than capable of forcing you to roll, then free hit for samus. The two best options if you find yourself in your shield is a) fullshield and if the samus is dsmash happy, you CAN shield grab her dsamsh or b) light shield and then get away from her. Learn to powershield, nothing frustrates a samus more than consistently power shielding her projectiles. Edgeguarding samus is your key here, get her off the stage, and keep her off. If she is high up there, throw eggs!!!!! They knockback can be DI'ed to help her recover, but it is difficult and the damage adds up quickly. CC dsmash/dtilt absolutely ruin a sweetspot up b from her, but make sure she doesn't come above the stage and catch you with it. Samus' dash attack is slightly better, don't try to trade off with it, instead try spacing your fsmash. I have been caught numerous times dashing at yoshi, and then being punished by an fsmash. Try charging your fsmash/dsmash out too, samus likes to spot dodge in this match up and those attacks are good punishers.