Looking on the internet deeply has found these results:

not too seriously is the most popular phrase on the web.

not so seriously 1651,000 results on the web	not too seriously 1861,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Do not take my joke so seriously! Enough as so not to take life so seriously. I'm not feeling so hot. Seriously. He is not so seriously ill Just put your head down, do the work, and try not to take it so seriously. I'm not In fact, there are still many countries that think they can escape this if they try to return to old times by saying that anything to do with the separation of infrastructures and services should not be taken so seriously. He is not so seriously ill l don't trust that fellow	She's wounded, but not too seriously. It seems to me that I did not take too seriously, my friend. One can only hope that the Seychelles are not too seriously disadvantaged by this unseemly delay. People tend not to take him too seriously. Not too seriously, just enough to get the hero thing going. You know, it's good not to take life too seriously. The Committee also notes that some Member States do not take these initiatives too seriously. And there's something kind of liberating about not taking it all too seriously.

not so seriously

1651,000 results on the web

not too seriously

1861,000 results on the web

More popular!

Do not take my joke so seriously!
Enough as so not to take life so seriously.
I'm not feeling so hot. Seriously.
He is not so seriously ill
Just put your head down, do the work, and try not to take it so seriously.
I'm not
In fact, there are still many countries that think they can escape this if they try to return to old times by saying that anything to do with the separation of infrastructures and services should not be taken so seriously.
He is not so seriously ill l don't trust that fellow

She's wounded, but not too seriously.
It seems to me that I did not take too seriously, my friend.
One can only hope that the Seychelles are not too seriously disadvantaged by this unseemly delay.
People tend not to take him too seriously.
Not too seriously, just enough to get the hero thing going.
You know, it's good not to take life too seriously.
The Committee also notes that some Member States do not take these initiatives too seriously.
And there's something kind of liberating about not taking it all too seriously.