Does no one really have a lot of advice or input on the ness MU? I kind of do, if its worth anything.
(My experience comes from playing with Dr. Grin, a local Norcal Ness main.)
50:50 or 45-55 that can be determined on specific stages. (i.e. D3 would do well on stages with decent platforms, and space like battlefield or dreamland. However, on CQC stages like fountain, or flat stages like Smashville Ness gets the benefit of being able to combo D3 exceptionally well. Honestly, he has to get in to do it, but ness has a pretty solid combo/punish game on D3. Most notably off of dash attack, and down throw. The best bet is to try to avoid Ness' approaches, and his PK fire activation by abusing your disjointed moves, using minions, tilts. Ness is also very susceptible to being gimped by throw--> fair/dair off stage. Also, his recovery is very easy to punish if you knock him to a point to where he has to rely on Up-b to recover. Reads with F-Smash, are very good to punish this attempt to recover, as well as down-b, or really anything you want that you can manage to snag the landing on stage. Keep in mind, F-Smash just covers ledge very well in that it can have large knock back with the tip (hammer head) or if the face of the hammer hits someone from below, it will spike them down, so it covers options that ride along an edge in an attempt to snap to it.
Waddle Dashing helps break the pace because all of Ness's approaches from the air rely on DJC typically, so you can reposition yourself to disrupt the pace that the ness is trying to set up. Once you condition the ness to a more ground game kind of pace, then that's where you're more than likely gonna take over the match up. his only way to really reach you is to hope to grab, dash attack, or pray for a smash attach to hit, which isn't even beneficial for ness unless the attack is a combo finisher, or if he hits you with it as a kill move on a read, etc. Once you grab ness, you can hurt him severely with being able to tech chase with down throw, or if you just throw him off the edge, you can intercept his DJ attempt with a fair or even a bair while he is rising up. Then you can choose to edge guard from the stage, or chase him further with more fairs, then recover.
At the end of the day, this is a MU where you want to make sure you take your time, pick your options, and punish relentlessly. The sooner you rid him of a double jump off stage, the easier the MU is because he cant do anything but try to recover. However, ness has strong hitting moves that are just about always guaranteed on you off of down throw, and back throw is still a threating move if he can grab you in the high 100 percent ranges. (Back throw kills at 160 on battlefield on the edge, even with perpendicular DI enacted. More moves you wanna watch out for will be his down tilt, and down air. D-tilt's fast start up make it pretty difficult to grab or CC war with him. Down air just acts like a pseudo falcon stomp, and is pretty much the same, its just smaller.
TL;DR: Don't get grabbed, try to avoid small stages, disrupt his double jump cancel timing. Gimp his jumps off stage, then do D3 things to kill him.