Remember a few years ago when it was chic to have glassware with the air bubbles trapped inside? It was supposed to show that it was handcrafted. The gear that I make is the same way. My beautifully curving lines of the stitching indicates the handmade effort that goes into each piece. Straight lines are for professionals and people who need to spend more time outdoors and less time worrying about stitches.
Here's the bottom line. Curved stitches: (1) hold two or more pieces of fabric together (2) don't degrade the integrity of the fabric or the seam and (3) don't look all that bad.
Be content that you are saving tons of money by sewing the gear yourself and most of all that you are holding a piece of outdoor gear that YOU made.