Air dashing is one of the best mobility options in the game. Yes he got punished for it sometimes(just like you do with any approach in the game) but he also get's a LOT from it as well, especially considering the strain on his opponents mind trying to react/predict the airdash and space accordingly. Rob can instantly cover space unlike any other character and that unique mobility trait gives him a lot of tools in the form of conditioning opponents/baiting approaches and other things. Now don't get me wrong I don't think Rob is top tier or anything lol I just think it's absurd to say he has poor mobility when it's one of his strongest traits.
You can't really react to it in a realistic scenario, but I never said and never meant to imply you could. Rather, the situations where R.O.B. can reliably air dash and attack are ones that are common to reckognize.
Now, to compare anyone to Fox isn't fair since he's good at most things, but I use this comparison strictly on mobility-
R.O.B.'s shortcomings in the traditional movement game (dashdancing, shffling, et al) and telegraphed nature of his air dashes (similar to Ness's DJC, starting one immediately close to the ground means that no aerial can come out before he lands), means that he can't pick and choose his engagements as a character like Fox, or Falcon, or even Roy can.
R.O.B. can move to most areas of the stage and can stay in the air for a while, but even JCaesar himself has admitted that trying to use air dashes to bait and punish or otherwise as a key mobility and approach option proves unsuccessful, and primarily uses them to follow up during combos, edgeguard, or, most obviously, recover.
I'd say on the whole that it is a relatively weak mobility option compared to what other characters have, and the fact that it is one of his strongest traits is not a good thing.