How do I rotate or flip a table in Numbers?

You can only flip and rotate objects placed on the sheet - such as images, shapes, text boxes, etc.

There is no option to rotate or flip an entire table

While you cannot physically rotate a table you can transpose the values in a table by clicking in it and going in the menu to Table > Transpose Rows and Columns.

Rearrange rows and columns in Numbers on Mac - Apple Support

SG