First things first, here are Ness's best tools. Most of his aerials, throws, PK Thunder and usmash. Nair acts as a good oos option and can KO later on. Fair is an excellent comboing, spacing, and edgeguarding tool. Uair can juggle and kill, and is one of the strongest uair in the game. Bair is a very fast killing move but rather situational. Dair is his worst aerial but it does have situational uses. Try to mix up your aerials and keep your opponents guessing. Now we have his grab. Dthrow allows for tons of followup, nearly all of his aerials. In case they airdodge, you can punish their landing with your smash attacks, PK Fire and sometimes PK Thunder or PKT2. Fthrow have strong bkb so it's good in sending your opponents offstage to edgeguard and it kills on the Smashville platform at 10%. Uthrow doesn't allow many followups like dthrow but it can set up uair/PK Thunder juggles. Bthrow is the strongest throw in the game, killing floaties under 80% near the ledge. PK Thunder is an excellent edgeguarding projectile. Should your opponent airdodge back to the stage, you can hit yourself to activate PKT2 and they die at 30%. Should they do nothing, followup with an aerial to rack up damage. Usmash covers a lot of distance and comes out fairly quick, so it's a good airdodge read tool off of dthrow.
Now for Ness's best stages. Smashville is his best - the platform allows Ness to recover high, aiding his Rosalina MU. And as mentioned, fthrow kills off the platform at 10%. Also since the blastlines are near, this helps bthrow to kill even earlier. Due to its relatively low ceiling, it also aids Ness's uair. Next is Halberd. Uair kills people around 80%. Delfino: Ness can 0-death using fthrow and fair combo on walkoffs. The island section also aids Ness's linear recovery should he fails to make it back to the stage. Bad stages: definitely don't go to Lylat. The platforms prevent Ness from comboing, and if he somehow falls under the main platform, you have nearly no hope to recover.
Ness's worst MU is Rosalina clearly. Her long range attacks make it difficult for Ness to deal damage to her, due to his short range, and Rosalina can just keep him out. To exacerbate this, Ness has limited options to kill Luma effectively. Also, when Ness is offstage, it usually means death for him since Rosalina can use her Gravitational Pull to absorb his PK Thunder. So I'd suggest pocketing Dark Pit for this matchup since Electroshock Arm instantly kill Luma ipunder most circumstances, and Rosa's shield isn't big enough to cover Luma. Sure you're gonna get punished but Rosa can't do anything off a grab since Luma is gone, and her KO power isn't great with Luma lost anyway. Additionally, Dark Pit has disjointed range and a good neutral and a projectile, helping him with approaching.
Ness's neutral is pretty mediocre due to his overall short range, hard time approaching, PK Fire being predictable and bad landing options.
Overall, Ness is a very good character and is balanced. Try to play somewhat defensively with him and go full offense when needed.
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Now to your playstyle. Your reaction time is pretty slow, so work on that. Don't dash while dthrowing, just jump up and fair. You'd have to be quick enough for the combo to be true, but it isn't hard. Also you seem to don't know his approach options; thry're nair and fair, situationally PK Fire. Also, work on your PK Thunder angles. Also learn various options while at the edge: edge hopping for example; drop from the ledge and immediately jump afterwards. Ness's aerials work best as a "get-the-f***-off-of-me" under those circumstances, but don't use it too much since his aerials have moderate landing lag. Don't use his smash attacks too much since they're generally not good. I noticed that your reading ability is good, so try to have better reaction times. Also try running shield to limit your opponent's punishing options on your approach. Also having better reaction times help when you perfect shield (shielding against attacks in four frames the shield is up. To notify players, it makes a "clang" sound and the shield will glow brighter. Perfect shielding allow you to attack immediately afterwards, so it's a good technique to keep in mind. Also I see you PK Thunder a lot, so decrease that and try to use aerials more.
As a Ness main, I'm happy to help! Contact me if you want tips! :D