Depends on your tiki release.
tiki1.9 introduces threaded style. You can check the differences between the styles (threaded / flat) on this site with the box style just above the box where you reply to a forum thread.
Depends on you style, some are better than others
You can check on this site by picking another style on the left column "style"
After you can change the appeareance by changing the css of your style
(for exemple if you are using moreneat style, you can change styles/moreneat.css). Some entries .subcomment, .commentstitle
If you like phpBB, have a look to tikipro.org