You might get better help asking the Yoshi forum, since punishing tends to be character/moveset specific.
In your case, you probably shouldn't try to punish rolls with F-smash. Yoshi's only has one frame that does damage, frame 14, so not only do you have to be 14 frames ahead of them, but the one frame that actually does damage has to line up with their vulnerability frames.
You should try more tilts and grabs to punish rolls at low percents. It's not a great idea to over-commit to punishing rolls if you're having trouble doing it. They are so fast. Be happy with your 7-8% of damage and hit them with something harder next time.
Something like down-B would be a better choice for a hard punish. It can be b-reversed to punish roll behinds. Since it is twice as fast to activate than f-smash, you'll be much more likely to punish a roll in-your-face more reliably.
edit - forgot to mention that dash attacks are generally good for punishing rolls-away. Especially Yoshi's which covers a samus-roll's length of distance. Punish rolls away with dash and rolls in with jabs, tilts, and down-b.