Normally I'm the guy who writes big critique posts on what to do differently or what tendencies I see that could be improved on in one's gameplay. I'm gonna try and cut out the word "big" from that and try and keep my comments short:
1ST VID:
-- You dodged a lot when you didn't need to. A habit that I picked up on when playing melee/PM was to space properly before thinking to dodge so that it's more of a fall-back measure. In this game, rolling and spot-dodging are great in that they have a higher number of invincibility frames (it seems like it to me), but that doesn't mean they're any more safe. Try using those dodge moves less and work more on maneuvering around the opponent's coverage
(eg#1. DDD has gordos to set walls up; those can be knocked back or reflected, and it helps if you do so. Trying to dodge around those puts you on a limited path, and limiting your movement is never a good idea.
eg#2. DDD has huge range with his hammer. Dodging around in that space before you're in danger of being hit by a move allows him to react and set up to cover your trajectory or position)
-- Know your secondary jump height. I saw @1:20 or so that you came short of the ledge and didn't PKT--> PKT2 early after maneuvering around the gordo. That could've been to avoid DDD hitting your thunder, but it looked a bit more like you thought you'd grab the ledge instead. Not only with recovery, but there was another instance in which you jumped over DDD into a position where you couldn't do anything to him; being above your opponent with Ness is generally bad, so keep in mind your jump heights when trying to move around your opponent.
-- You seemed a bit TOO passive. I know that DDD is to be feared in the same manner as Bowser or Ganon because they hit hard, but there were times where you stood around or backed off when you could at the very least get a FAir, DashAttack or DashGrab in. This is about recognizing the lag time between moves and the speed that Ness has to punish or capitalize on small mistakes in spacing or movement. I can't really think of a good example, but I'll try to look for one later.
2ND VID
-- PKT isn't a good neutral game move. If both of you are on equal footing, it's easy for the opponent to react and either shield it or run in and hit you with an attack before you recover from lag. I generally use PKT to cover other characters and their path around the stage when they're trying to return to the ground. Otherwise, they can shield which basically negates that as a reliable option.
Nothing else is really a big issue. You have decent combo knowledge, though sometimes you simply missed a move. One small thing is to vary your followups when your opponent gets trapped in a PK Fire pillar, but that's not as big a concern.