Yeah, the Sheik matchup is pretty bad. Falco is far more difficult (in my opinion) than Fox because the kid just hates lasers that he can't run through. A defensive Marth really sucks. The biggest reason Marth is not as strong as he was in, say, 2005 is because people decided to start baiting out the sword swing. He can't do anything while you are in his face. Bait out the fair and start comboing accordingly. Be patient and content with not approaching him unless he does a dumb thing.
Fox and Falco can be combo'd really easily. The shine pressure is real, so with smart play and liberal use of shielding, you can combo them better. Sometimes a Yoshi mentality is necessary because Ness is so weak as a character. Getting hit is sometimes the better option, but since your recovery is not good, limit it of course. I almost had my shield broken the other day, but then I got out and combo'd the Falco lol.
Also, when you play Ness, you will normally have the crowd because nobody really sees Ness played. So you get the support, and not many people know the matchup very well. A Fox player let me go Final Destination because he had no clue how to DI Ness's grab and did not know about the combos from them. It was a good game. XD
Just have fun dude.