Hitler stated he viewed the Sioux and by extension the majority of tribes around at the time as Aryans, so presumably he was cool with their existence at least. He used to read lots of westerns and especially ones sympathetic towards Natives, so it’s unclear where his views came from but likely that.

Though he also used the US reservation system as an inspirational for his ghettos and early models of subjugating races before extermination, but I think that was based on its efficacy rather than who it was targeted at.