If I was getting married, I'd have no problem with a guest wearing that … but I'm not fussy about that kind of thing, and I'm sure not every bride would feel that way and some might think it was a bit much. I think it's the combination of the lace and the off-white colour that people might have an issue with. The colour would be fine if it wasn't lace, and the lace would be fine if it wasn't so close to white, but both together might cause some brides to get a bit twitchy.
If you're close to the bride and have a good relationship with her, you could always ask. I wanted to wear a black dress to my friend's wedding and lots of people said I shouldn't, but in the end I just asked her and she laughed and said it was absolutely fine … in fact, she said 'It would actually seem wrong if you wore any other colour!' as it's a standing joke among my friends that I have a very black wardrobe.