My first striped dress came from a supermarket, four pounds something, navy and white, and I wore it until the stripes on one sleeve had gone slightly grey from being the one I always grabbed. What it taught me is that stripes hide a multitude of sins on a cheap jersey, puckered seams, a hem that's not quite level, but they can't hide fit, because a line that's supposed to run straight across your body will show you exactly where the dress is pulling or gaping within about two steps.
Getting the size right matters more here than on a plain dress, because stripes are basically a grid printed onto fabric, and any grid tells on you. Go up a size if you're between two and the stripes bunch anywhere, especially across the chest or hips, they'll kink and break the line instead of running clean, and that's the tell that it's too tight rather than just fitted. Horizontal stripes on a bias cut can also twist slightly as you walk, so I always do my usual run through before I commit, arms up to check the sleeve doesn't ride and expose more wrist than I bargained for, sitting down to see if the hem creeps past what I'm comfortable with, and a turn in front of the mirror to watch whether the stripes stay level or start sliding round the body like a badly hung picture.
Where striped dresses let me down is anything with a dropped shoulder or a very fluid, floaty fabric, the stripe pattern loses its crispness and just looks blurred, which defeats the point of choosing stripes in the first place. They're also not the one to reach for if you need real structure, a cheap striped jersey in particular has no backbone and will cling exactly where you don't want it to by midafternoon.
A good one ages well though. Cotton or a proper cotton mix keeps its shape wash after wash, the stripes stay sharp instead of softening into smudges, and the colour holds, which matters because a faded navy stripe looks tired fast. Wash it inside out, cold, and skip the tumble dryer if you can, heat is what kills the crispness that makes stripes worth having. Mine from three years ago still looks properly striped, not like it's been left in the sun.
The mistake to avoid is buying a striped dress in a size based on how the stripes look in a photo rather than how they sit on you in person, because that grid will find every inch you were hoping to disguise.