Some Americans might celebrate Indian festivals. It all depends on their family backgrounds and/or their friends. Most Americans love to celebrate, but there are those that would prefer not to celebrate with friends.
As for Indian festivals, specifically, if someone lives near a festival, they might go just to see what’s going on.
If someone has a friend that has Indian heritage, they might celebrate with their friend or at least wish them a happy festival. The food, the atmosphere, and the time of day could also contribute to someone celebrating an Indian festival if they are not of that background. Otherwise, it might not appeal to someone or they might not understand why someone celebrates on a specific day.
This doesn’t apply only to Indian festivals, but since this question specifically asks about Indian festivals, I have focused on those festivals.