Gist of the whole thing. [Edit: this suddenly turned into a bio of his SSB4 competitive history lol. Refer for only the first, second, and last paragraph for the short response. Will put in a spoiler tab later.]
Ness was top tier early on due to the big hype his significant buffs from Brawl to SSB4 brought to the table. His pretty troublesome matchup against Rosa, Villy, and Sheik, as well as Diddy and Sonic being annoying as well, where then noted, and Ness then dropped from top tier to a solid high tier, with the player base and results to back it up.
Then at the 2nd half of 2016, the top Ness players then disappeared all at once, only to return at the end of the year. During this time, opinion on him declined even further. Although Ness loved the nerfs to Diddy and especially Sheik, he doesn't like Rosa remaining prevalent, the new Corrin being introduced, and Marthcina being heavily buffed into viability.
There was also the whole potential craze going along in the Smash community. Unfortunately for Ness, Lucas is one of those characters, with Taiheita's 17th at Smash Con 2016 adding fuel to the fire. So even when Ness' results returned back to form in 2017, as well as Taiheita's results in Japan declining, people commonly put Lucas over Ness for the potential factor.
Fortunately for the character by the end of 2017, Taiheita's retirement pretty much ended the big potential craze for Lucas, so Ness' results became more recognized by the Smash community outside of top players. Ness' improved results in 2018 thanks to players like Gackt and BestNess also came as well. He never achieved the results of back then, but still pretty good. Makes me wonder where he would be if there was one last SSB4 tier list, because the latest one from Dec 2017 has him paired with Lucas at 28/29th (potential craze or not, I have no clue how Lucas made it over DH and Link, but ok then?). He is generally considered upper mid to high tier at the end of SSB4's metagame.
Going into Ultimate, one thing some top players complained about Ness in SSB4 besides from his recovery (which is kind of silly considering that they live in a Cloud-infested metagame lol) is that he lacks depth as a character (despite Lucina, Sonic, and MK being a thing). Well Ness has plenty in Ultimate, thanks Magnet based movement now being a thing. His new up air has a bit of a learning curve as well. Giving from his already high standing so far in Ultimate, he has more room to grow rather than having the metagame slowly slip from his fingertips.