So to be quite frank, Ike won't make it as a solo-mained character. He should be switched out against the following:
-Cloud
-Diddy Kong
-Sheik
Against Cloud, he loses hard on platform stages. Battlefield and Dream Land do him no favors, and Lylat Cruise is just a death sentence.
Against Diddy Kong, he loses harshly on stages, but may do damage on platform stages.
Against Sheik, it's always been an uphill match. However, there has been limited testing on post-patch Sheik. But historically, this has been Ike's worst matchup.
In times like these, it's best to switch to another character after the first round. If you and your opponent agree to Smashville, Town & City, or Final Destination against Cloud, you can very much win. Cloud doesn't do as well on flat stages. However, he will murder Ike on platform stages, where it is best to switch out. Personally, I secondary Cloud, so I have no problem going ditto against another Cloud main. Once the second round passes, I either win the set or it's Game 3, where I can pick an advantageous stage for Ike. If it's a Bo5, I stay Cloud until I lose.
Against Diddy Kong, it's a point to start with Ike since it's your main. Ban the flat stages, leaving Battlefield, Dream Land, Town & City, and possibly Lylat Cruise. Final Destination doesn't do you any favors, as Diddy has camping range for days and can catch your landings, and Smashville does a disservice to Ike because Diddy can use the floating platform to save himself from your edgeguarding. That being said, you do better on FD since Diddy isn't as fatal at catching landings as Mario or Sonic. If you can win Game 1, switch because the Diddy will be taking you to disadvantaged stage.
As for Sheik, it's still a mystery to me as I have not played a Sheik in bracket to gauge whether or not we can win this matchup with her nerfed weight and our patched UThrow Fair confirms.
Cloud and Mario are both effective against at least 2/3 of these characters. If you're going to use a secondary, it's best to find someone who can cover and sort Ike's worst matchups. Mario himself is also a detrimental matchup, but he is clearly manageable.
Also, some matchups, like Jigglypuff, are very annoying for Ike, but managed much easier using Cloud with the same strategies. Ike loses proper use of his powerful moves, like USmash, as it takes only one mistake for Jigglypuff to Rest you at 53% and end the stock. Cloud uses very similar spacing tools, but takes less commitment. His Nair is safer and his DTilt is a slide that can cover the distance to punish Jigglypuff effectively. Moreover, Cloud's Uair can kill Jigglpuff around 115% and his Dair COMPLETELY shuts down any and all of her attacks as it has a huge, lingering hitbox.
Finally, Cloud's limit charge forces Jigglypuff to approach. She is not the fastest character, making it that much easier to charge limit. This turns into Limit Cross Slash, which kills Jigglypuff around 80% and outranges the character completely.
Cloud's one weakness is getting gimped. It's a weakness he shares with Ike, but his Up B doesn't have a projectile like feature to throw Jiggs off. However, this can be averted simply by staying midstage and charging limit to force the approach.
Another example is Little Mac. While Ike wins the matchup, Cloud no doubt has a superior advantage over the character. DTilt slide can punish Little Mac on shield and his Nair is a very FAST attack that has sufficient knockback.
If you can help it, find your secondary and sort the matchups you struggle with. Ike can still do well against certain characters, like Pikachu. But his unignorable weaknesses turn him into a struggle if you begin losing percents and stocks. Right now, his biggest strength is literally his ability to confirm UThrow Fair for kill confirms, while his second biggest strength is the upgraded Fair itself, which covers a larger range and can kill off the ledge around 90% to 100%, making it one of the single most effective edgeguarding tools in the game. To match that is his Bair, which comes out faster. Against characters, like Pikachu and Diddy Kong, who tend to "disrespect" by hitting you before the Fair frame start, you can just use retreating Bair when they get up from the ledge with Fair or use it as an approach option.
However, Ike undoubtedly loses the neutral to a number of characters on the roster. To be quite frank, he still needs buffs in other areas before I see him being viable as a solo main. As more people learn the Ike matchup, they're learning exactly where he's punishable at - both in neutral and offstage - and making it harder and harder for Ike mains to take stocks.
One notable weakness is Dair. It's very slow, and still not as reliable for spiking as Captain Falcon's, Cloud's, Charizard's, or Donkey Kong's. Another is the lack of special options in neutral. For certain characters, grabbing Banana Peels or Gyros still gives you plenty of fighting moves with B. But Ike is highly limited, forcing setups and reads for using them effectively against the other player. I worry it's similar to the Brawl scenario, where he goes 50-50 against most of the roster, 55-45 against a number of the higher tiers, and 60-40 against several others, offering very limited advantage against the roster outside of fundamentals, reads, and good player judgement. The main thing will always be spacing against the opponent, strong defensive play, grab combos, edgeguarding, and carefully managing recovery.