My initial thought was that people were over-thinking this. "Dress to impress" to me just seems like an offhand way of saying "wear something nice-looking", rather than anything sinister.
However, if the girl whose party it is is into the Roblox "Dress to Impress" game, then this is a very bad theme to put on the invitation. There are several probolems.
First, the game itself is horribly shallow and encourages an unhealthy obsession with appearance.
Second, choosing this theme excludes people whose parents don't let them play the game.
The third issue with a Roblox Dress to Impress themed party, though, is that it isn't really a theme at all. Depending on the theme in a particular session of the game, players could be dressed as anything from a farmhand to Marie Antoinette. So, you could wear anything and meet the theme.
Fourth, there's a risk the party girl will have decided a more specific theme for the party with her closer friends (just as there is a more specific theme in a Dress to Impress game) and those not meeting it end up feeling left out.
Fifth, the game involves players voting on others' outfits and overall appearance. This is relatively harmless in the game as everyone is represented by a similar basic avatar and you're judging the players' choices of how to decorate the avatar. But if the party degenerates into "Dress to Impress IRL" with children judging each other's looks, this could get very nasty.