So sorry to hear about your PND. I think it's hard sometimes to feel you look good when your head just isn't there.
I had a complete clothes panic before going for my birthday lunch last week, trying on my whole wardrobe, declaring it all hideous and fat making and feeling I couldn't go. In the end I just wore the first thing I tried on and went out feeling dreadful. Half an hour later I forgot I looked hideous, but it is a horrible feeling.
It's probably me being shallow, but sometimes if I can get the outfit right I feel the more confident about whatever I have to do that day. And if the outfit is lovely, but I look grotty, then it doesn't matter so much.
I don't think you should be so hard on yourself. You had a baby a few months ago and it takes a while to get back to normal.
I know your body is different now, but what sort of style did you used to feel good in? Is that dress something you would choose. I think it's very age appropriate as long as you get the accessories right.
I think the minimum benchmark is if you try on something you already own and feel ok in and then try the dress and you feel it's more flattering. Do you have someone in RL who you can try it on around who will give you an opinion.
Do you have time and the will to take care of yourself a little at the moment? Not suggesting the ubiquitous spa day, but the usual eating right, bit of exercise, bit of grooming. For me recently a proper skin care regime has made a world of difference to looking grey in a really short time.
I hope I am not being too trite here as I don't really understand what you are going through with PND, but I do know quite recently what it feels like to feel post baby like I have lost my looks and my spark and am working on getting it back.