Nasality and twang are not the same thing. They may, indeed, seem or sound similar, but they are definitely not the same.
Nasality is the sound we hear when a singer has his nasal port open. Is this good or bad? Well, I guess that depends on what kind of sound you want to make with your voice. For the most part, nasality is not considered an esthetically-pleasing sound; however, some singers may indeed do this and consider this their signature sound.
Twang on the other hand is an important coordination that every singer should learn and understand. The ability to “twang” creates particular frequencies in the voice. There frequencies add volume, brassiness, brightness, crispness, and/or fullness. This is an important coordination that when used with other vocal coordinations gives the singer freedom to express themselves dynamically and with texture.
Twang may sound unpleasant in its’ purest form, but when added to the voice in varying degrees, allows a voice to be interesting and believable.