I feel as if Robin isn't as good as people make them out to be. Their attacks are easy to punish, most of their attacks can be reflected/shielded, and their attacks can't KO under 150%, not even their Smash Attacks or Thoron will KO the opponent at that damage (trust me, I've tried numerous times already).
This is objectively false. Levin back-air is incredibly powerful. Levin up-air is godlike and will kill even with perfect vectoring reasonably early. Levin Fair can kill around 120-30% closer to the edges. Thoron will kill around 130%, and can kill ludicrously quickly if you're smart on off-stage snipes. Arcthunder can kill below 150%, as well. And all of their Smash attacks will kill inside the 100-150% range (and occasionally even earlier, in my experience), so honestly I'm not even sure what or how you tested this to lead you to that conclusion. Robin has two meteor smashes, one of which is incredibly safe (that is, it's his/her recovery) and an incredibly powerful edgeguarding game between the utility, speed, and trajectory (almost completely horizontal) of neutral air, fast and strong aerials (with high priority and good disjoint), and obviously a huge variety of powerful projectiles. Robin has more killing tools than a few other lesser characters combined.
And I don't get what's 'easy to punish' The smash attacks? Obviously. What aren't? Forward tilt is a fast and effective spacing tool, down tilt is an incredibly fast poke, and the start-up of jab is fast, as well. Arcfire and Thunder tomes have start-up, but if you're just throwing them out while your opponent is close enough and has the means to punish it, you're being an idiot. The endlag on Arcfire only punishes bad spacing, and even missing it creates a wall that your opponent can either roll through, jump over, or wait out, and all of those are situations that Robin can prepare for and deal with. Maybe they're easy to punish if they insist on going hard on an approach, but Robin was never built for that kind of strategy. You're not punishing Robin so much as poorly optimized play.
Yes, a lot of their specials can be reflected. That's not as huge a deal as you make it. It makes match-ups like Palutena much harder, but Robin still has tools to deal with it, and a reflector-happy opponent is a situation you can easily take advantage of regardless. And the fact that so many of Robin's projectiles force your opponent to shield (that is, Arcfire, Arcthunder, and Thoron) is absolutely a
good thing. A shielded opponent is open to a grab, and while Robin's throws (and grab) are far from great, they're still useful for setting up position, racking up damage, or going for follow-ups depending on percentages. They still force your opponent into a situation that Robin can control.
This is ignoring Nosferatu, which while definitely situational, only adds to Robin's fairly robust mix-up game, and has the distinction of being a command grab, therefore able to be utilized in the air.
Plus, it goes without saying that the speed, distance, and power of Thoron makes it arguably the best charged projectile in the game. Even the threat of simply having it loaded can provoke your opponent into making mistakes. There's barely any character (if any) that has the means to totally disrespect it.
More recently, the removal of vertical vectoring/reintroducing of DI also vastly improves the vertical kill power of Robin, making Levin up-air even stronger.
They're not perfect; definitely not top five in my experience. But Robin's versatility and potency cannot be underestimated at all. They have the means to create opportunities, force opponents into making mistakes, and capitalize on them with vicious efficiency.