Was pakistan any better at tests before the arrival of 2020?
better than what they are right now :)
We also need to realize that the spike in T20 cricket also coincided with the lack of international Test cricket due to the security situation in Pakistan. So while other countries were still playing test cricket, Pakistan had a huge gap of almost a year and a half or may be more, and yes, during this time, our team was playing more T20 matches. So obviously, the team got into a T20 mindset in general.
So I wouldn't say that T20 is the "cause" of our failure in the test arena, its just that we've played more of it lately due to lack of opportunities for test cricket. Had our team played a couple of more test series in the last two years, we would've been faring better.