I assume that if a person is seeking a critical viewpoint, they will have said something to indicate that. Something like, 'What do you think about . . .', 'is this border color okay . . .', 'need input on such and such . . .', 'please let me know if you see something out of place . . .', 'how can I match seams better . . .', etc. If someone hasn't indicated they want critical feedback, I wouldn't volunteer it.
If I'm really struck by a quilt, I'll comment that I think it's beautiful, etc. Thankfully, tastes vary widely, and every quilt is going to be found beautiful by some of us!