The first time I did mattress stitch was on a raglan and I was thrilled how it turned out. I suspect that the chunky yarn may have something to do with your dissatisfaction. Maybe you can try with a thinner yarn the same color?
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry