Hi,
Depending on the version of Moodle, yes you can create a theme without a parent theme, but this would mean that your theme would need to carry ALL the CSS that the default theme carries.
Prior to Moodle 3.2 all the themes that used either Base or Bootstrapbase themes as a parent theme, worked OK. The themes that did not use a parent theme still incorporated the CSS from Base or Bootstrapbase depending on the layout.
Making a theme of your own still needs the relevant CSS styles that make Moodle work. So in this respect it makes better sense to make a child theme, with changes made to the layout, for which you would just need to add your own CSS.
Moodle 3.2 is different as there are other ways of making changes to the Boost theme which is now the Default theme in Moodle. These changes can be via a child theme, or a Boost theme 'preset' file which just changes the look of the Boost theme mainly using colours and images.
I hope this answers your question?
Mary