Marth is very difficult due to the fact that if you want to win you have to tipper his moves,there is no room for error with Marth.He also had a bad recovery.
MK has many different styles that is required in his playstyle,especially vs campers and being that he is very light with terrible air mobility.
Diddy is pretty easy since you can do many things out of his dash attack,and you can glide toss bananas all over the place.Diddy is one of the most frustrating characters,not to mention his fthrow and bthrow can put the enemy wherever did wants him.The bad thing is if he doesn't have his bananas by him it can spell disaster.
Olimar is pretty simple : camp and pivot grab.He has many options and can't be completely shutdown,he can juggle like nobody's business and his smashes are projectiles.His whistle is amazing as it gives you super armor allowing you to survive kill moves.His Pikmin have very strange abilities though,Purple is the most feared Pikmin.(Red = aerials) (Blue = fthrow or bthrow) (Yellow = stronger smashes) (White = fast and rack damage,throw or grab attack) (Purple = give hitstun,mow over,very strong smashes but less range).
Low tiers are not easy to pick up,as they are difficult to win with.
DDD can dthrow chain grab,with his amazing grab range and spam bair to keep them offstage.He does have a projectile and can last to godnasty percents.
Wario is fun,there is no certain way to play him - just don't get grabbed (air grab release sends Wario straight upwards so it's a free hit for the opponent).Also don't double jump too much,he has the best aerial mobility and his fsmash murders.Not to mention he has the strongest move (waft/fart) which kills extremely early by takes about 2 mins to charge.While he has his waft ready it puts extreme pressure on the opponent.