The short answer is no you can't decompile the DLL file. However, you really don't need to decompile the dll because you can simply download the source.  Make changes to the source, recompile and then just copy your files back up to your host.  You will want to backup your web.config file though.

Also I would suggest to you that you don't really need to edit the source at all.  If you are wanting to remove elements that point to DNN, then you can simply edit the Skin you are using.

Generally skins are located under /Portals/default folder.

You might also want to look at the documentation for Dotnetnuke and how it works.  There really isn't ever a reason to modify the core of Dotnetnuke, unless you are adding a functionaility that can't be included as a module.  

I think if you start looking at the skinning guide, available in the Documentation download you will find everything you are looking for.

Stuart