For some reason, this situation reminds me of the Bible passage in Luke 20:45 about the widow's mite. I think it's the part about the teachers of the law and the flowing robes that is speaking to me.
To me, what you're encountering sounds like an inflated sense of self-importance. There's an implicit assumption that you should take heed of her wardrobe advice. She may even believe that she's being helpful. Although I may be reading more into this story than what's there, it almost sounds to me like she judges whether people are church-worthy based on their attire. I have no doubt that this judgment occurs in many congregations; however, that she would feel compelled to voice it is something else entirely.
Of all the responses I've seen, "How kind of you to take an interest" is the one that seems most appropriate to me. I'd follow that with a dose of bean dip. With strong eye contact and tone, it could put a stop to her intrusions. If it doesn't work the first time, repeat as needed. I would suggest not engaging her on this topic or attempting to defend yourself. If you show complete disinterest, she will find it less interesting to accost you.
Last, there's the issue of her approaching you during Communion. While her topic of conversation is unwelcome, the fact that she's interrupting your Communion meditation could be considered a bigger picture problem. You could address that with her rather than the topic of clothing as well.