It is just semantics, the difference from high tier 29th and mid tier 30th is minimal and having tiers inflated with a ton of characters seems lazy and half hearted effort wise but if it's accurate then that's what really matters.
What I did at the end of Smash 4 was based it primarily on matchup spread. High tiers would have few or no -2 MUs, a few -1 MUs, and going even with at least a few top tiers. Meanwhile, those at the top of high-mid would likely have a few -2 MUs holding them back, and at lower ends up high-mid they'd have more -2 MUs in general.
Since I think good tier lists are primarily matchup based it also means that there will be limits to how many characters can be in each tier. Every character cannot be top tier because in order to be top tier you'll have to win against X number of characters (generally most of the non-top tier cast). I think that one of the distinctions between high tier and high-mid is that high tiers generally have 0 to 1 really bad (-2) matchups, while high-mids generally have a few.
This makes early tier lists highly speculative, since our understanding of matchups are rather limited. In a sense, results do affect tier lists, since top level results help us understand which MUs are good and which are bad. For instance, in Smash 4 it was long theorized that Corrin and Captain Falcon went even, but Fatality did incredibly well against Corrin mains, which indicates that Captain Falcon wins the matchup (and both Fatality and Cosmos admitted as much near the end of Smash 4). Another example is Rosalina vs Cloud: Early on some Cloud players claimed that they lost to Rosa, while some Rosa players claimed they lost to Cloud. In practice, it seems that Cloud does, indeed, beat Rosa slightly, in Smash 4 at least (pretty likely that he does in Ultimate too though, but I haven't studied the matchup in Ultimate).
With all this being said, is Ness high tier? It's fairly plausible that he is. His worst MUs from 4 are not as bad as they used to be, although he still has a bunch of -1 MUs at least. Does he have any -2 MUs? That's a good question, and I don't know the answer to that yet. At this point in time, we have early impressions that could turn out to be incorrect. It seems likely to me that Ness still struggles with characters like Lucina, Cloud, Rosalina & Luma, and Sonic, although whether they are -1 or -2 MUs I don't know yet. I suspect, in general, that there'll be fewer -2 MUs than before, since the game is better balanced now. Any tier list at this point in time is highly speculative anyway.
Is Ness high tier? It's plausible, but it's also plausible that he isn't. Smash 4 Olimar got good results despite being a high-mid tier character, after all, as did Toon Link, Villager, and Duck Hunt (and other characters as well, including Ness). I also know that Ness players in the past have overrated their character, in Smash 4 at least: In 2016, many thought he was top 15, in 2018 he was down to around top 30 (which I think ended up being more accurate). Wasn't his results even better early Smash 4 than they are now*?
*Granted, more character variety in general, but FOW got some great placings in early Smash 4, and he was ranked #12 on the first PGR, with incredible placings such as 7th at EVO 2015 and 5th at GENESIS 3. I hope we'll see some Ness players getting top 8 at majors since I find him a fun character to watch and he does seem a bit better than in Smash 4.