Sure!
The main thing is the difference in getting kills. Dedede will often live to 160 or 180. Sheik can die at 80 from dsmash with some rage, or maybe 90 or so from bair.
Sheik can hit Dedede easily, so she can usually wrack up damage quickly, but struggles with closing the stock. So Dedede will spend a lot of time with rage.
It's true that Sheik will be hitting Dedede way more than he hits her, but you have to look at how many hits Dedede needs to kill. He can get about 20% off a grab until she's at about about 50-60%. It's not too tough for him to get grabs, especially if she fairs a lot (she only needs to mess up the spacing a little). She can't just run away and needle because gordo beats needles and does 14%. If Dedede starts predicting aerial approaches he can stuff her with fair which is like 12%. Dedede just has to win a fraction of the exchanges and he can get her to kill percent first. His jab and dtilt can box her out ok, but she can punish them so Dedede will need to be able to predict somewhat. Jab to grab plus grab follow up is like 25%. At kill percent powershielding a fair gets him a dsmash, which can actually kill as low as 60% near the ledge with a lot of rage.
Dedede is really hard (as in almost impossible) to gimp. His dtilt can hit her when she appears from vanish to grab the ledge.
Sheik will pretty much be dominating a lot in the match cuz Sheik vs fat, but it only takes a few slight mistakes/reads to get her to kill percent, whereas she has to get him really high in percent without making mistakes. I know some people say matchups should be based on optimal play, but everyone makes small mistakes/gets read. That's just being realistic.