You can call him whatever you want, but all I know is that Ness is too competitive of a character in this meta to call a mid tier. I think we all understand Ness' strengths, so IMO we should pinpoint Ness' specific flaws as a character to start some discussion on how we can work on advancing his meta. I'll start with some.
1. He has mediocre landing options. This is due to a combination of poor air speed (though good acceleration), lack of an aerial protecting the bottom of him (d-air is only good for autocancelling), and lack of a burst movement option (like Bouncing Fish, or Monkey/Flip Kick). Here are some of his options:
a. Ness obviously has a great air dodge, but it is now second nature for most to punish that option because it is so overused. They wait for it to come out, and then swat him away with an aerial.
b. Ness' double jump leaves him even more vulnerable on the way down, making it very risky. Sometimes, this just works anyway (it is important to take risks sometimes), so don't rule it out completely. PK Jump can be interesting in some situations as well.
c. PSI Magnet is useful as a stall, and players like FOW have used it to great effect. More Ness players should incorporate this move into their game as a mixup.
d. Ness' aerials are not very good for landing (though they are amazing in other contexts). The following aerials are safe on shield: N-air at perfect spacing, U-air with decent spacing, and B-air with decent spacing. However, because Ness has to be low to the ground to utilize these aerials safely, they tend to be easy to power shield or predictable in timing, making them difficult to actually land. Autocancel d-air is another mixup.
e. Going for the ledge is honestly one of Ness' best options to land; however, with how easy it is to punish ledge options in this game, it can be a bad idea.
f. PK Thunder 1 is another mixup; it should be used from a height from where it can autocancel, however.
AND THAT'S ABOUT IT. (unless I'm forgetting something) None of these options are very reliable, so landing is clearly a challenge that Ness players need to work on overcoming.
2. Ness has a bad recovery. A long time ago I went on a rant (where I talked about his recovery a lot):
here. Scroll down to the part where I start listing recovery mixups (for the record, I don't really stand by the rest of my post, I'm trying to be more practical as much as I love Ness). Of course his recovery is not really great by any standard, but it is still something we can optimize. Master every mix up and make it a chore to gimp Ness!
3. He has some very rough matchups.
As of right now, I'm of the opinion that it is not worth playing as Ness vs Rosaluma. Pick up a secondary for her, it's necessary if you're serious about winning. Secondaries I would recommend are either MK or ZSS (my personal choice). ZSS has a vaguely similar playstyle to Ness, so I would recommend her. She beats Rosalina very solidly, and is not that hard to learn. Another tough matchup: Sheik. It is not impossible to beat her as Ness (though if you are picking up a secondary anyway, you might as well pick them vs. her). FOW and Shaky have had good results against top-level Sheiks, but the matchup is still clearly bad. Some other matchups like Villager are uncomfortable for Ness players, but not necessarily out of his favor. Again, with a secondary (which I find again to be necessary for Ness to succeed), you can pick and choose which matchups you want to play.
4. He has bad mobility. This is kind of hard to get around at all, as it's not like he can just magically increase his dash speed. To compensate for his bad mobility, Ness players are forced to throw out aerials in neutral, which makes it hard for opponents to challenge his space. By virtue of his bad mobility, Ness has trouble approaching in general (though once he finds a combo opener, expect major pain). Overall, his bad mobility makes his neutral predictable, which is a very major flaw in my opinion. We Ness players need to reinvent Ness' neutral to be less reliant on aerials and find what is safe and useful on the ground.
Those are the 4 things holding Ness back. Yes, they are big flaws, which are not particularly indicative of a top tier character. Nonetheless, I think everyone should consider that the way Ness is played is largely the same as how he was played within the first 3 months of release; in other words, his meta has not advanced very far. We have absolutely amazing players like FOW and Shaky who are out there getting great results for Ness, but the fact of the matter is that Ness has not been pushed as far as he can be pushed. We need to work on addressing these flaws and working to circumvent them so that Ness players can be even more successful. Overall, I think Ness has what it takes to succeed in spite of what's wrong with him.