depending you could hit them out of the air with bair and finish with a dsmash << lol
also uair is surprises me when it kills sometimes..
oh, and believe it or not, down 'b' can be a pretty good aerial killer if the opportunity presents itself.
there's really not A LOT of killing opportunities with Yoshi (though still quite a bit imo), you have to be a beast yourself and make opportunities, this isn't Fox Sheik Peach Falcon etc..
like you could get gutsy, something that almost always works is if you hit someone in the air, stand directly below them and don't do ****, instead of hitting them with a nair just stand there and neutral b, a move without much lag but seems like it has a lot, then when your opponent double jumps at you with a dair or something, wavedash away and fsmash them as they land (though I personally use dash away to pivot fsmash). These are some of the mysterious mindgames I'm told this game has <<