Kind of.

In this context saying someone is "off the reservation" means that all Natives were stupid/crazy or uneducated. So if someone did something stupid, it meant they were acting like a Native from a reservation. Like how someone might say "hood", "ghetto", or more recently "huddy".

In some context "you're off the reservation" was a veiled threat to Natives meaning they have no protection and will likely be assaulted if they kept acting "uppity".

Just not how it was just used in the comment you replied to.