impossible, fireballs bounce. yoshi's jabs can only shield part of his body. They can still land above him.
Try spacing just so that you can hit them with jab, and that is not even counting the tilts.
It's really rare a chance you can't space right to cancel it with jab, even when it bounces.
You have to remember that while mario is throwing fireballs, he is also approaching you. So if you use a move to ping out his fireballs, yoyu become defenseless and vulnerable to mario's attacks.
Ok so, let's do this by 3 pieces: 1-Remeber the mario is aprroaching you, 2- ping out his fireballs, 3- becoming defenseless and vulnerable to his attacks.
I wanna start with number 2, and I hope you'll understand why: By cancelling I don't mean making an attack go TRHOUGH it, I meant an attack cancelling WITH it, and that's why I chose the jab.
Then there's numer 3: if you actually meant what I said before, I can't really understand how cancelling a move actually makes you vulnerable, since once both moves are cancelled you can do any move you want (even roll if you wish, but I wouldn't recommend that) that is a ground move. Cancelling doesn't seems to have any lag if done correctely, and even if it does, it's so little it doesn't matter.
And then there's the number 1: Mario is approaching! OMFG Run AWAY!!!
Jokes aside, mario IS approaching, fact. But is that really a bad thing? I would like to recall once on the Ivysaur matchup discussion we had long ago. We said back then we would wanted it to approach. Because once it did approach, we could read it's moves and counter as we pleased. In fact, the fight was pretty much decided on this: You attack first, you die. He attacks first, he dies. It's almost the same concept here, but now it's just a little harder to see: If you try to go all out with mario, you lose (and by that I mean a GIANT KICK IN THE), but what if you can see his attack coming somehow (it can be from a short hop that get's him in attack range)?
I say counter him. Don't try to run, or shield, or dodge away to the sides. Find a way to make his attack to be punishable (pivot grab maybe, or maybe an Usmash).
Fact is, as it was stated on this very thread, yoshi has more options approaching than mario, and mario has safer approaches. Safer approaches if looked separated from the context of the fight. What I mean is, even the safest of approaches, if countered the right way, is not completely safe. So that does mean you can find a little hole in his approach that comes after fireballs. And since we have MORE approaches, means more options to deal with his approaches after fireballs.
So now you may ask, how am I supposed to know when this guy is going to attack?
That's when it gets kinda hard. When you cancel the fireballs you are actually baiting him to attack. After that it's just quick reflexes in work.
I can actually say I fought some pretty good marios here, and that jabbing the fireballs, as stupid as it may seem, works if you know how to do it. Of course, if anything goes wrong with your cancelling or spacing you're pretty much screwed.
Thats my in-depth response, Love ya! (PS: don't get mad at me for being kinda stubborn or even for having big replies, I just wanna be sure I'm doing my best)