Rockbox is an alternative firmware which is made to be more configurable (have more options) than the original firmware music players come with. So yes, Rockbox adds a layer of complexity to almost everything.
Since Rockbox menus are not customizable, not even with themes (they only change the looks, they don't add or remove functions or menu items), you would have a lot of work trying to simplify things. You'd need to compile Rockbox from source and remove the features you need - it can be done, but it's not trivial specially for people who never touched source code before.
What themes can do for you is simplifying the WPS (while-playing screen, the "Now Playing" screen), so that it only shows the very relevant information (e.g. the title of the song and it's artist, the volume level and the battery charge). Themes can also change the user interface in order to change the fonts used, make the font size bigger, change the icons (or remove them), the colors, and the position of things in the screen.
Some of these things you can configure on the player itself, others you'll need to change on existing themes or create a new one from scratch.
You can also delete all the plugins, that will remove many of the "places" to get lost. The plugins are things like games, utilities (text editor, calculator) and demos (which are of little usage, except perhaps PictureFlow). All the plugins come installed with Rockbox, you can't download additional ones - but you can delete the existing ones.
Tell us if you need any more help, for example, removing plugins or changing themes.