I'm on the autism spectrum too and I really don't feel comfortable lying. So here is what I do when someone asks me for my opinion and I know that they won't like the answer:
Instead of answering "I don't like this dress", I try to understand why I don't like it. For example, if I don't like the color, I can say:
I'm not a big fan of this color. I find it too much for me.
This color makes me think of < insert awful thing here >, so no, I'm definitively not a fan.
There are several important points with those phrasing. First, the "sugar coating" with "I'm not a big fan". The truth is "I really hate this color", but being less categorical won't hurt the other feeling as much and I'm comfortable with minimizing how I feel (which I find different from lying).
Another point is the "for me" and the "I am". Taste is very personal and insisting on how it is your taste and not a universal truth make the other person feel less judge about the fact that they do like it.
You notice that I gave two phrasing example. I find the second one to be even better than the first one because you are not simply saying that you dislike the color, you are also explaining why. In my experience, people will more easily accept your judgment if they know why you feel this way.
For example, I don't like food with mint in it because it makes me think of toothpaste. People might not feel the same, but they can't argue with how you feel (even if my dad roll his eyes every time I say that I don't like mint because of that).
For the dress, if the problem is not with the dress itself but with the other person wearing the dress, again, try to understand why you don't like it before saying anything.
For example, it might just be you that isn't accustomed to seeing them in something like that. In this you can just say:
I'm really not used to see you inside something like that. It's weird.
In this case, you are not saying that it's bad or good. You are just stating the fact that you are not accustomed to this and thus, it might take you some time before liking it.