THEORY FIGHTER TIME!
Ok, so here's why Sinestro is probably his worst matchup
1) Their playstyles are extremely similar. Both character can zone, can operate up close, and both are really damn good when trait comes out. Difference is that not only does Sinestro take control of the air better, Sinestro is capable of getting out trait more often. Furthermore, once Sinestro HAS trait out, he can keep it out, compared to Zod who should get trait out more regularly, but can't because Fear Blast > Zod's tools.
2) While both are fairly similar, Sinestro's zoning is VASTLY superior to Zod's. Fear Blast aside, Boulders give Sinestro a way to knock Zod out of his MB Zod Balls, and the Tires give great trait time. Side Arm is not enough in that match like Harley Guns or Nightwing Blast might be.
3) Sinestro runs better without trait than Zod without trait. This is PURELY opinion and I acknowledge it, but Sinestro is closer to being a complete character solo than Zod. Zod with trait out is more dangerous than Sinestro with trait out, but the trade off is the 10+ second CD and stupid long startup on the Ghost of Thomas the Tank Engine(see recently made Zod thread on Zod trait glitch for context). Ordinarily this isn't that bad since Zod controls the match against most characters well enough to justify it, but as said before, Sinestro's own ability to control Zod's options heavily reduces the chance that Zod can get trait out. Zod's Ghostless the majority of the match.