I don't know what she brings to the party, but this is likely one of many red flags she's been waving. Just because you go through with the wedding doesn't mean the flags vanish.
Edit: just noticed the date on the sign. RIP, Greg. Good luck to you.
I also think it would be a funny way of directing the bride's guests and the groom's guests. The bride's will be directed to "Laura's Wedding, Featuring Greg," and the groom's to "Greg's Wedding, Featuring Laura."
It's a red flag of Greg doesn't like it. Could have been a red flag if Lauren wasn't sure how Greg would react to it when she showed it to him (mostly dependent on how she would react if he didn't like it). Same if any of the above happened with roles reversed.
But yeah, odds are Greg and Lauren both find it funny.
How do you know Greg didn't make the joke? Do you now Greg and Lauren? Did you give them a gift off their registry or did you do the right thing and give them cold hard cash (via check)? Did you tip the bartender a 20 to start the night and say "remember this face!" and proceed to order thirteen G&Ts over the course of the ceremony?