A Tiefling is not an aligned outsider because it is Native to the Prime Material plane.
This is good logic. Unfortunately logic takes a back seat to rules. The rules for the native subtype are as follows:
This subtype is applied only to outsiders. These creatures have mortal ancestors or a strong connection to the Material Plane and can be raised, reincarnated, or resurrected just as other living creatures can be. Creatures with this subtype are native to the Material Plane. Unlike true outsiders, native outsiders need to eat and sleep.
The subtype specifically states the creature is still an outsider (because only outsiders can have this subtype).
You could further argue that the subtype specifically states that you are a "native to the Material Plane". But what does that mean? This page defines many effects that natives have on a specific plane. But it does not define any special rules for a native from the material plane.
We know a native outsider on the material plane is not considered "extraplanar" for spells such as dismissal etc. And we know that "person" spells do not affect tieflings/aasimars due to their outsider nature.
Is there any indication that a native outsider is not an outsider?