First I have to say that I like Chrysta Bell's Tammy.
But yeah, I don't understand the "this is what Lynch wanted" defense at all. If that is acceptable in this case, why not in other cases too? Whenever someone is acting poorly in someone's opinion it could always be what the director wanted.
I'm sure that if we look at the movie and tv history and see what scenes and performances these people think are bad, many of those scenes and performances are actually just the way the director wanted them to be. It still doesn't mean they couldn't be seven as bad.