Robin is the least mobile character in a game where sheer mobility trumps a lot of other attributes.
Wrong. While Robin's ground speed is the overall lowest, their air speed is slightly above average. Robin's OVERALL mobility is not the worst in the game.
Robin has unspectcular frame data in a game where frame data is an important facet of a character.
Again, wrong. Robin doesn't have a single move with more than 16 frames of startup. The main kill moves/damage rackers/combo starters are jab (4 frames), grab (7 frames), fair (11 frames), bair and up air (9 and 10 respectively). That's really not bad.
Robin's moves - apart from his projectiles - all have a good deal of commitment to them - in a game where safety is king.
First of all, like 80% of Robin's game is projectiles, so discounting those is kinda dumb. Second, Robin does have some safe things. Fire Jab and down smash are much safer than they appear. And to add in projectiles, retreating ArcFire is actually somewhat challenging to punish. All of the Thunder spells should be harder to punish now.
Now add those issues together and you have a character with glaring holes in his gameplay. It takes quite a bit to make up for that. Does Robin have what it takes? I don't see the amazing footsie options in neutral [grab jab and tilts are all pretty lackluster], damage out put or even recovery that could help Robin overcome these weaknesses. More reliable KO options is a huge deal though by itself not enough to make Robin a good character.
This is what makes me think that you do not know how to play Robin. Robin's Jab is absolutely fantastic, with the wind jab racking up as much as 21% on fast fallers. Even if the opponent DI's out of the final hit, Robin won't be punished for it. And the jab kills at around 130-140% without rage. Fire Jab kills at around 120% near the ledge and starts working when characters can fall out of the wind jab. The new grab buff means that Robin kills everyone from down throw at kill percents, and the grab leads into combos and traps at every relevant percent. I'll give you that Robin has bland tilts, but Robin more than makes up for it with their aerials, with all of them killing sub-120%.
Robin's damage output is also one of the highest in the game. They are hitting you for +15% for every exchange. ArcFire/ArcThunder practically guarantee +20% and lead to killing moves! I can't see any argument for this one. And ElWind is not this godawful recovery that everyone thinks it is. Robin can drift quite a bit between the two hits, and it can spike those who screw up offstage trying to edgeguard Robin. With the combination of ArcFire, Robin's air speed, and ElWind's height, Robin should be able to safely recover most of the time.