Does a grounded Koopa Klaw even connect reliably? I feel like I never see people use the grounded version. I would think the CC setup could work if they do getup attack. But you would not be able to cover the wakeup roll options on reaction. Maybe if they roll behind...
The way I'd do it... if they intend to roll away, they usually do it instantly. By then you are usually too slow to reach in time, you can then use a fair or a koopa klaw to read a jump or a spotdodge.
If they roll back well wavedash back pivot ftilt or pivot grab lol, you are usually in range, and they do it as soon as they land too. Bowser is big, so if you stand in front of the grounded character you can punish by turning around.
As for the KK on the ground... yea the range is bad and it's slow. I think it would depend on the character. But I know that they often try to instantly jump or spot dodge. You can read this with either a KK or a fair once again. If you see that they tend to bring up their shields, a normal grab can do too.
Even a ftilt can be good if they are high percent. Bowser have so many options. I guess you have to adapt to your opponent's habits.
Edit: Oh and btw, if the KK grab misses but they spot dodge, the strong hit will hit them, sending them up.