Those nerfs are heavily exaggerated.
1) The mechanics of Ultimate has buffed him a lot. He now has a ground speed you have to respect (he didn't get buffed more than the other chars in that department), and the ability to dash cancel is a godsend to a character that utilizes tilts very heavily (yes). Down tilt in particular has it's ability to combo improved (not really), and the nerfs to shielding makes it a good poking tool (albeit not Chrom down tilt levels of good poking). I think the fact that people doesn't put up shielding as often as in SSB4 is a buff more than a nerf, as opponents are no longer inclined to block Kirby's button. (no, Kirby never struggled with shields, most of his stuff is safe on shield including D-Tilt, he loses to hitboxes. and d-tilt only offers reward when it trips, which is 35/40%)
2) As you said before, the up tilt strings is a mere psuedo-counterpick against fastfallers. He does better against them than other low tiers in SSB4, but it is still tricky. In Ultimate, his up tilt has more range (NO)(which compliments the above-mentioned dash canceling) and the increased knockback of the up tilt makes it easier to connect to his other aerials (No, jumps are just faster.), potentially putting offstage (which I will get to later). This makes it more applicable to all matchups on not just fastfallers. (His follow-ups on non-fastallers are the same, he just lost his follow-ups against fastfallers)
3) Down air is actually much more potent in this game due to the mechanic changes, and that is where Kirby's biggest strength is: he has one of the scariest edgeguarding games in the game. Edgeguarding in SSB4 was harder for Kirby as you can airdodge off of everything. In this game, you only have one airdodge, which means that Kirby can infintely followup after your airdodge, which makes down air a frightening option (No, it's a small frame18 hitbox, still loses to attack and airdodge, he just lost the reward on it). If the spike doesn't instantly KO, Kirby can easily insert another one (??? no). Almost all, if not all, does not do very well at all against Kirby offstage (only those who recover low without a disjoint = not a lot). Onstage, Kirby can still easily followup opponents into down air. If they airdodge away, you get a dash attack KO for your troubles. (you have to read it, if they dodge your grab they're out, you have to land a frame18 unsafe move AND get a read to kill at 150% great)
4) Kirby's grab did indeed get nerfed, but grabbing in this game in general is not the best option. This would've been a big blow to him if it was SSB4. Granted that it does affect him negatively, but it shields away other applicable options he can do. His aerials has got a good cut in landing lag (pretty much at Melee Puff levels), to the point that he can pressure opponents in defense and get great followups from it.(he could already do that in S4, now people just don't shield and beat him) Again, the shield mechanic changes and improved grounded approach (as well as remaining a small character) make it an option you have to respect (although against swordies, it is harder to do). (against anyone that understands that attack beats kirby)
5) Up throw was only slightly nerfed in KO power. It wasn't the most reliable KO throw anyways. (it was very reliable, and a "slight" nerf when it comes to killing is really big, especially if you grabbed under a platform and didn't kill because of the nerf, you're ready for another 30% to deal)
6) I keep hearing that Copy Ability is easier to get knocked out, but I am not really seeing evidence that it is true. From the changelog lists I've seen, when he loses the copy power is unchanged from SSB4. (the changelog isn't reliable, ask litterally any kirby player if the copy is easier to lose)
7) Basing it off of stuff you only found in two weeks and finding better success with other easier characters to play (although the characters you did select are much better characters) is flawed reasoning to say that the character is terrible. You can say that you found better success with Donkey Kong than with Peach because DK is a much easier character to play, but only two weeks have passed and Peach has more depth. (OK but I'd have seen the depth if I played Peach and wouldn't have given up on her if there was hope)
The only reason why he is one of the most underwhelming characters in the game (not arguing that he is bottom 5), is because he still have low reach and his air speed is still relatively small. (it's not relatively small, it's abyssmal. but OK) The fact that swordies that swing their swords like empty water bottles lurk in the upper tiers, does not help. But he is not a bad character, just outclassed by the fact that the entire cast is decent at worst, while still having issues. (if everyone is better you're a terrible character, unless the game is perfectly balanced [it obviously isn't])Mark my words though: when Kirby gets a buff to his moveset's range and his air speed, watch him shoot up the tier list (I don't see it happening, but buffing his air speed would help a lot, and range would be amazing)(if his feet had Brawl range, he lowkey would be at the upper high tier)(no).