It's not so much the grab itself, but who it's on and what you get out of the grab. Mewtwo and Ness have rather average grabs, but Ness gets a good setup off of Dthrow and Bthrow is a stupidly good kill throw (please nerf that ****), while Mewtwo gets 2 good kill throws as well. ZSS, widely considered a top 5 character, has a tether grab (read: one of the worst grabs in the game), but she's also very good outside of her crappy tether grab and iirc gets good reward from grabs.
Robin has a lot of strengths:
-disjoints
-(powerful) zoning tools/projectiles
-excellent kill power
-a command grab
that heals
-some combo/kill setups in Arcfire/Arcthunder/Dthrow
-good damage per hit
.....but she'll never be top tier for two reasons: mobility and frame data. Smash is, as I understand it, a game ruled by mobility and safety. If you look at all the characters generally considered to be "good"/common tournament picks (
, etc), you'll notice that almost all of them have above average mobility or better. With the exceptions being Rosalina and Luigi, because Luigi's Fireballs and insane grab reward and Rosalina's great Utilt/Uair/Dair+Luma being nuts happen to, for whatever reason, override their so-so mobility (and possibly a couple of their other issues). And you'll also notice that their isn't a single character with good mobility that's generally considered to be low tier.
Frame data is somewhat less important but is still really nice to have. Or least you want to have very good frame data on a handful of your moves anyway. But when you lack a quick get the **** off me option and several characters have faster jabs and tilts than you it kinda hurts in competitive play.
Shiek in particular stands out to me as an example of "mobility and frame data>everything else". She has very few, if any, disjoints, her damage per hit is garbage, she hits about as hard as crippled old man, is lightweight, and lacks a command grab. And at this point buttloads of people have grab combo so whatever. Yet almost everyone considers her the #1 character right now, and what does she have? Great mobility and arguably the best frame data in the game. Oh and needles, but she'd still probably be good even if needles were super nerfed.
Of course, it's not so simple as that, and many other factors effect a character's viability/tier placement (
has excellent frame data, ground moves, and speed, but is held back by his ****ing terrible recovery, lack of a good air game, and lightweight, while
has hitbox issues and Super Missiles should hit a little harder,
is
, and the things noted above about
and
), but I don't think it's crazy to say that mobility (and frame data) is hella important in this metagame and that Robin's strengths don't
quite substitute for her poor mobility and mostly average frame data. Though its not like you'd wanna give her significant mobility and overall frame data buffs, because then we'd have
Grima a monster on our hands and that's no fun.
So then the question is, if not top tier, how high exactly can
go? Possibly very high, but I sure as **** wouldn't really know. I suppose we will know in time.