I think Robin's criminally underrated, many people place him/her smack-dab in the middle of mid-tier but I attest that
no, that's too low. The problem I think lies in the way the community is conceptualizing the game. Frame data and combos are still super important (and Robin actually has those in useful quantities by the by), but the area where Robin really shines is the capacity to capitalize on reading the opponent, a clear emphasis for this game particularly, due to its buffed shielding mechanics.
That might seem kind of dumb at first, but hear me out for a second. Obviously when talking about character ratings you have to keep in mind characters in context; just because one character is good at (and indeed, seems built around) capitalizing on good reads doesn't necessarily mean that they're any better at doing that than the current favorites for top tier. Sheik and ZSS both have crazy mad punish games and combos that are super damaging and super rewarding, dealing upwards of 2 billion percent converted off of one good read, give or take 50,000. Okay, that's obviously hyperbolic, and also inaccurate:
Nairo's 0-death "combo" at SSC recently wasn't even a combo in the traditional sense, but a chain of smaller combos strung together by a series of resets; grabs, nairs, anything to smack Falcon right back into hitstun
where he belongs so ZSS could continue racking up damage. This is how high-level Smash 4 play looks-- it's not necessarily a technically impressive feat, but requires absolutely astounding mental dexterity and a deep understanding of the game systems at large-- in other words, it requires the player to read their opponent in order to play effectively.
Enter Robin, a character whose literal in-universe job it is to read his/her opponent and formulate tactics accordingly. One of my favorite things about Sakurai as a director is his ability to translate non-action-oriented experiences into action-substantiated systems, and Robin is no exception to that-- Robin requires a keen mind for tactics in order to play effectively. Laying down traps, catching the opponent out of rolls, baiting airdodges, and alternating appropriately between zoning and aerial rushdown are Robin's bread-and-butter. If the Robin player fails to do these things with consistency, Robin gets heavily punished for his/her poor mobility, predictable recovery, and huge vulnerability after moves. Unlike current favorites for top tier, Robin can't just throw out moves to apply pressure because nearly every tool Robin has is punishable.
I know it might seem like I'm being super down on this character right now, but I wouldn't be arguing that he/she was underrated if I didn't have a "however" to throw in, and speak of the devil, here it comes. However punishable Robin may be, the reward a skillful Robin player can get out of a single good read is
enormous. Levin Sword attacks deal damage closing on on 20% each hit and LS aerials come out quickly and, due to the disjointed hitbox, outrange many current top tier darlings by a significant margin. Robin can challenge Sheik's fair, s/he can challenge Falcon's bair, and though s/he can't really challenge Rosalina's uair (really, who can?), s/he can escape it with a well-placed Elwind. Just mathing this out for a second, and given how early LS can kill, that means Robin effectively puts most opponents in kill percent in about 6-9 hits. That's
Ganondorf damage, only we're not Ganondorf here-- we're fast (in terms of attack speed, not movement, as addressed previously). Couple this with how much conversion potential Robin has out of his/her projectiles (Arcfire into LS aerial, arcthunder into Nos or grab+dthrow for either a jab or aerial follow-up, so on and so on) and you're looking at a stock that, played well, could be ended in the neighborhood of three good reads. That's not an insurmountable task, and that's just assuming the player is
only dealing damage off of reads, completely neglecting Robin's huge zoning potential and ability to contest many if not most approaches with priority, aerial or otherwise (wind jab basically acts as a wall that stops anything from getting in, ftilt comes out fast and definitely beats out barehanded jabs, retreating fair or nair are even better). Additionally, Nosferatu allows Robin to correct for any mistakes that are made by allowing a skillful Robin to just erase that damage from the game forever-- poof, it's gone, I can still JV2. And did I mention the spikes? Robin has two of them, and they're both reliable, at that!
I think primarily the biggest problems facing Robin are that s/he's a hard character to learn, and so not many people are playing. As a result, the Robin meta is only slowly evolving (contrast to Diddy Kong, who being similar to his Brawl incarnation had 6 years to develop a dominant metagame strategy-- at least before the nerfs messed with that, so people say). But, hey, Raziek did pretty well at EVO (despite having none of his matches streamed), so I'm optimistic about the future of this character.
I ain't sayin' Robin's top-10, but... Probably at least top 11. ¯\_(ツ)_/¯