I Think it's unfair to look at a couple big games by opposing players and say he is a bad defender. He didn't play great D in some of the games you mentioned but overall his defense has actually been very good this season.
Here are some metrics:
NBA.com player tracking:
Stephen Curry allows 39.6% shooting to his opponents when he is the primary defender. That's 3.5% lower than those players would normally shoot on those shots.
In the 243 possessions where he has been the primary defender on a pick n roll ball handler he allows 0.82 points per possession. That's in the 49.5 percentile for that play. So he is basically average.
However, he is third among PGs in real plus minus.
None of these numbers are perfect but 2 of them say he is a very good defender. He may not be great against the pick n roll because he isnt the biggest, strongest guy but he has done a very good job rotating on shooters, communicating on switches as well as reading opposing defenses and disrupting ball movement with deflections and steals (he is one of the steal leaders in the NBA currently)
The biggest drawback is his lack of size and strength. If you had Russ (for example) and it was optimal to switch him on a big, HE could fight the big for position and deny the ball for at least a few seconds and that's often enough to force the opposition into doing something else. If Curry is switched on a PF, you pretty much have to double or just give up 2/FTs.
So i would say Curry is actually a pretty good defender overall. Not all world but very good to great team defender and an average one on one defender whose biggest drawback is his lack of strength.