Mixing past simple and past perfect.

IMO as a Brit, there are all poor, some worse than others.
Primary school rule 1.... Don't use the verb 'get' !

Replacement suggestions...

The grades he obtained were not as bad as those obtained previously by his brother.

Your 'tense' question would require a better example.

It's common to use get in American English. In this case, get means the same thing as obtain. I don't know what a better example would be or I'd have used one. I wasn't trying to write a "good" sentence. I want to know if it matters when I mix tenses like that. Whether it's unfavorable or changes meaning/implication in some way.

She wasn't as sad as he was about Trump's win.
This sentence is fine, I think, whether it could be written better or not.
She hadn't been as sad as he was about Trump's win.
I think this sentence is grammatical too, though I'm not sure, but does it mean something different than:
She hadnt been as sad as he had been about Trump's win.
If they don't mean something different, or if that nuance is indiscernible to native speakers, which is the preferred sentence between the two and why?

Nothing 'matters' except the social context in which a sentence is presented. 'Correctness'( or more accurately 'acceptability') whether grammatical or semantic, is subservient to that context. When a non-specific context is assumed, then it makes sense for the producer of the sentence to aim for the highest acceptable standard he can. ESL teachers may or may not be aware of such a fundamental point.

(Sorry...last comment should have been to you.)