While ESRB is necessary, it does impose limits on what game creators are able to do in terms of sexual themes and violence. Thankfully, it is not as strict as the rating systems in other countries, but there are still instances where games have needed to change their intended ideas to meet the requirements.
It might limit the creativity of developers to an extent, but I really don't think that game developers need extreme violence or sexual situations to be creative. They are already allowed both in games and generally the violent and sexual games tend to be the most uncreative. Creativity comes from having unique ideas, not how much "adult" material you can squeeze in a game.
ESRB ratings limit who can buy certain games, but it doesn't limit the creativity of game developers. Rating requirements limit what can be shown in games for children. Ratings obviously limit what parents think they should buy for kids. Much like movie ratings, kids won't see rated R movies nearly as much as PG-13 offerings. In the same way, M for Mature games may not sell as much as T for Teens due to the content of the game.
I do not believe the ESRB rating requirements limit the creativity of game developers. If it does, then they should rethink their involvement with worrying about these ratings. As an American consumer who has purchased many video games over my life time I can tell you I've never once paid any attention to these ratings and I don't feel like developers should either, unless they are specifically trying to release games in certain rankings. This doesn't necessarily stunt creativity however, it just presents challenges.
We've seen plenty of examples in the past (Manhunt, Grand Theft Auto) that show the rating systems warn about most things but outright ban very little, game developers are not being limited in their creativity by the ESRB system. While certain choices may change the rating on the front cover, they won't prevent the game from getting published.