Well if the damage is guaranteed I don't see why you would sacrifice free damage to go for a read that might not even work considering DL's hitboxes and how much of a vertical contrast they are to Corrin's Fair trajectory, at least, not in most situations, especially considering that DL's near-perfectly horizontal trajectory won't be killing (relatively) earlyish from an aerial string unless you're near one of the side blast zones. At percents when DL will kill a little further from the side blast zones, Corrin's Fair may (well, probably) send the opponent out of reach of such a DL read at which point the only viable option would then be to follow with a Uair, and that's only assuming that the knockback of Fair at such a percent wouldn't send the opponent too high for DJ -> Uair to miss afterwards.
In the case that Fair -> Uair isn't guaranteed (which is certainly possible, considering that iirc in the FE Colosseum footage Corrin gets Fair -> Uair on Bayonetta but connects because Bayonetta chose to jump away, and you can see the Uair hit Bayonetta mid-double-jump), it might exist as an option, in the sense that something like rising FH Fair -> DJ (likely slighlty backwards for tipper spacing) on the post-hitstun-airdodge to possible DL could be an option, but again that depends on a number of factors such as Corrin's DJ jump height and her aerial mobility, momentum-change properties of DL, to what extent Corrin's Fair can be DIed, Fair's natural trajectory, the relevant frame data for such a sequence to be successful, etc.