@Tiyo Kamtiyono -

These are actually two different things: Theme Unit Test Data, and Theme Preview Data.

Personally, I would like to see the previewer use the Theme Unit Test Data, but I understand the reason why it's not: the Theme Preview Data are intended to showcase the Themes and make them look beautiful; the Theme Unit Test Data are intended to challenge/break Themes for testing.

But agreed on the point that the Theme Preview Data could use improvement as well.