If a kirby just sits in place and ducks, I like to dance around his range, but not commit to anything, in a way that I can't be punished, at a range where he can't just react to what I throw at him, then he starts to stop ducking, maybe shield, maybe roll or spot dodge
Like, the only thing you should be committing too is walling or running options incase he starts to approach
but at this point, he has to stop ducking, if he isn't ducking, he is just a normal slow character and do what absol said
If he insists on ducking, keep pressuring him in this way, but keep not committing, if the kirby keeps insisting on not leaving the ducking animation, then hit him with something he can't react to, he is literally giving you a free hit, he is being reactive, so again, just don't let him react
but throwing things at a ducking kirby is really risky, I want to emphasize that, so only commit to hitting him when you are sure that he is being reactive, kirby will win just by damage output if you guess wrong, BUT dancing around him has almost no risk if you do it safely and don't commit, but the ONLY purpose to committing with really fast options that beat out reaction time is to make them know that they cannot get away with ducking too much
IDK this is how I play this matchup, I used to help another good sheik practice this matchup with my kirby, and I have played a few good kirby players with my sheik, I am no expert, but this DID work for me really well