Not necessarily as long as you are just adding basic content themes and things like that. I no very little about PHP and I have built a Joomla Site and have had it for years.
Pretty much no knowledge is required unless you want to go into creating your own themes, templates. or mods for it.
Joomla is kind of an insult to people like me who code our own applications from scratch in PHP and put hours into it when something like Joomla can be installed with the click of a mouse.
