When doing the nyah exercise for mix voice is it supposed to sound very nasally and have like a piercing quality to it

Absolutely! That's the result of nasal resonance and the vowel shape. Exercises like that (twangy/bratty/nasal) help singers maintain cord closure higher in their range, which is needed for mix!

Does that mean we shouldn't raise the soft palate when in mix voice 

It depends on your desired tone! You can also train mixed registration with a "dopey" MUHM or BUHB, which might lend itself to a lifted soft palate. But if you like the NYAH/NAY sounds for mix, then a neutral or lower soft palate will help. 

I don't get how you can sing In mixed voice if the soft palate is raised, isn't it necessary to have nasal resonance when in mixed voice 

I'm with you, George! I do think that mix usually has a lowered palate and some nasal resonance. But it's possible that some singers can remain in mixed registration and lift their palate a little bit for a warmer sound. All I'm saying is: I wouldn't rule it out, and I don't want to make a blanket statement.

I do typically introduce mix to beginners with that overly nasal sound, and it helps a lot of people to think of nasality as the "bridge" between their chest and head voice. For me personally, my voice favors a brighter, more head-dominant mix sound. But I want to be careful about prescribing one way to mix, since there's such variety among singers!

Hope that helps clear up any confusion!

It does thank you 

Great!