My dude, you used fsmash too many billions of times. For every one that worked you got punished 10 times more. You need to work on weening yourself off that move and instead use safer options because against a good player, a laggy fsmash is one of the free-est, easiest punishes that everyone hopes for against a Marth. ESPECIALLY yoshi, because that character relies on his shield game more than any other character, so he's already prone to just blocking your moves. Retreating short hop fairs, d-tilts, a wavedash back while looking for a reaction, or even plain dash dancing to force something - just don't stand and fsmash because that move sukz gargantuan ballz.
As for Yoshi, a few things I gleaned from playing one - you can't uthrow combo because yoshi is stupid heavy for some reason. Try for tech chases with fthrow and dthrow instead. Also, all his double jump cancelling nonsense can be beaten by counter. As far as I know the only way for him to even do that is with an aerial, so obviously he's gonna try and hit you...just counter it! Until they wise up of course. Like crimson said, nair is godlike against yoshi, so practice up on doing quick, safe nairs, including running, retreating, yadda yadda. At low percent, if they have horrible DI/don't crouch cancel you can even combo like 5 nairs in a row. What a great move. Also, you don't have to go in on his shield so much. Remember that his shieldgrab sukz so you don't have to worry about it and can space aerials all day on his shield until they slip up and start getting antsy.