That dress is not a wedding dress. Your analogy with a graduation gown falls flat on its face. Although (a) even there if you can't tell the difference between a graduation gown and a cape, I think you've got problems and (b) my mother wore capes more than she wore coats and she actually did wear a gray cape to mine. No one was confused or undermined.
No one could look at the OP's dress and think '" is she the bride? Why she's wearing a wedding dress?" As for "undermining the bride" - good grief. I'm running out of grips.
There were plenty of guests at Megan Markle's wedding wearing paler, non patterned dresses.