That seems about right.
I've done a total of 10 rescues so far (both event and jolly), and I've only gotten 3* and 4*, 50-50 for both of them.
Of course, my experience does no justice in displaying the actual rates, but I stopped there, because spending any more money was (I thought) pretty much useless when 5* cards were being given out in events like candy.
I remember I said it before, but I'll say it again. Currently, the Jolly Roger Rescue holds some of the most powerful cards. Even most Jolly Roger 5* outclass the Event 5* by far. Rarity isn't everything in this game.
I think the only real "top tier" cards that were given out during Events were Explorer Neria and Light Admiral Harpo. Sea-hand Ria might join their Ranks as well, especially if we get access to more copies. One can also make a case for Small Whirlwind Malina, considering she has the lowest Cost of all 5* with comparable ATK and Skill. But overall, I still don't consider her in the same group as Ebony, Garrett etc. Which can only be obtained through the Jolly Roger Rescue.
This game defenitly treats its "free" players very well, but if you want to become THE best, you will
need Jolly Roger Pirates.