Well, that cinches it, the thread is definitely showing through. Thanks for the interior picture. By the way, very nice work there! I am a fan of creasing the pockets as you have, and the border of the stitching to me makes it pop.
As you know, as soon as the cards go into the wallet, all sorts of lines and bulges are going to happen. Just a normal thing. So I don't know that I'd sweat too much over it. If it really bothers you, go with even thinner thread. Since it is only securing the bottom edge of a t-pocket, it isn't going to get a ton of stress. Just the cards being pushed down. Use glue also if you aren't already. I assume you are.