I've stood in enough changing rooms holding a smock dress to know the real question isn't whether it suits you standing still under shop lighting, it's whether it still works once you sit down, reach for something on a high shelf, or walk into a gust of wind outside. A good one survives all three without you thinking about it once.
If you've never owned one, start with a plain cotton or cotton blend version in a solid colour, something that stops around the knee or just below, with a seam or gather at the yoke so there's a bit of shape across the chest before it loosens out. Avoid anything with a huge, busy print for your first go, it hides the shape of the garment as much as the shape of you, and you want to learn how a smock dress actually sits before you take that on. Once you've got one, it pairs easily with flat boots and tights from autumn through to a cold snap in spring, and with trainers and bare legs the rest of the year, which is about as much range as any dress gives you. Add a long sleeve top underneath if the sleeves on the dress itself are short, or a slim jacket over the top on a cooler day, since the loose body means it has no warmth of its own to speak of. In high summer, in anything but the lightest cotton or linen, it can turn into a small greenhouse, so check the fabric weight before you commit to wearing it in August.
Sizing is where most people go wrong, because the loose cut makes it tempting to go up a size for comfort, and that's usually the mistake. Too big and the whole dress reads as shapeless rather than relaxed, swamping you from the shoulder down with nothing to anchor it. Too small across the yoke and it pulls at the chest even though the rest hangs free, which looks odd rather than fitted. Go true to size and let the body of the dress do the loosening, not the size label. Watch the hem too, since a smock dress with no waist seam can look very different sitting down than standing up, riding up at the front if the fabric is thin and has no weight to it. A well made one has enough drape to swing rather than cling, proper topstitching at the yoke and seams rather than a single flat run of stitching, and buttons or ties that are stitched through the fabric rather than glued on and gone within a few washes. A disappointing one is see through in daylight, with a hem that twists to one side after a single wash.
Reach for it on a day you want to be dressed without being gripped by anything, and skip it on a day you need real heat or real structure.