first off, the only smash you should not use when they are low is d smash, the others are perfectly fine for whenever.(except against DDD and climbers). F smash is a great spacer and a great "back the **** off move". up smash can usually be fallowed up by an up tilt at 0-10%, has little cool down lag, and is your best friend against wario.
second, DK's jab is one of if not the worst jab combo in the game, it should never be used regularly. its best to use a grab since it leads to WAY more options. you can use that or you can use a d tilt instead of a jab since it is just as fast, less punishable, and can lead to another move in most cases especially if you trip them.
if you want to beat a diddy just watch some of my videos.
http://www.youtube.com/watch?v=FU5uZDh9070
http://www.youtube.com/watch?v=kliw6s3De2g
http://www.youtube.com/watch?v=WrVxNqmXy3U
http://www.youtube.com/watch?v=upuhHuV0ak8