Just guard every seppuku so they become entranced, repeat them, and suffer more life loss. Basically if it has fire, you have to practice a counter to it.
236B for instance, can be easily ukemi JG'd, and is a somewhat used move against more inexperienced players.
44KB UB is inexcusably easy to JG. A buddy of mine who didn't know how to JG at all learned by starting with this kind of stuff. Learn the timing in practice, then apply it to a match. You'll find yourself doing it, even in high-octane games.
8A+B takes practice to deal with, but what I essentially go for is to just get up if I was knocked down, relax, and try to JG the shit out of whatever is coming. 8A+B has so many variations, that JG just wins in terms of risk factor by miles. Even stepping has a commitment level to some of the cancels Yoshi can do here, so I find calming down and attempting a JG (if you are above 50% success rate or so here,) will net the best defense all round.
BT 2A+K will never catch you off surprise, because you will expect it in nearly every BT Yoshi shows you. Expect this move by watching the rounds and life bars. A gutsy Yoshi will usually see your low health, especially if he has the life lead to survive. If you will both die, but the score is 1-2, he has no reason to require a life lead. I don't expect this move much outside those scenarios, but a clutch JG here by becoming aware will net insane momentum. Always make sure to throw a back-step as soon as you land it in case he goes for broke with 2A+K AAAA / A+K. Raph can input 4

44A+B to effectively take advantage here.
66A+K is pretty much the same, bar BT. Even Indian stance A+K has JG as a catch-all in terms of timing. It shuts down nearly every option after the move. Super dragon-fly A+K is another rare one, it has such a ridiculously tell-tale animation, that if you don't practice it, your muscle memory isn't even needed.
44B should be treated with - I SEE FIRE, JG ATTEMPT. Seriously, if you see the animation start, just hit JG straight away. As long as you aren't hugging a wall, 44BA will whiff, and you can then 4A+B, or 44BB happens and you JG catches it.
I find B+K a risky choice here, but I guess B+K - JG is somewhat reliable seeing as 4BB is popular. It's just that JG-when-fire-appears helps you if you misread the 44B and it was another seppuku attempt. If nothing happens and he is charging his shit, move out of the way slowly and comfortably as he takes the next 30 minutes for the animation.
I'm not saying JG is some god-tier defense... much, but Yoshi DOES have some annoying A+K options that are dealt with hard thanks to JG. It's also a great character for beginners to try to JG due to how easy these moves are to see coming thanks to their animations. Due to the sheer amount of them though, a lot of Yoshi's get them off too often on the unsuspecting.