Go for it! Since you've never moved out, you should have an idea of what type of things might be of special importance to your mother. Be respectful of those areas. However, be ruthless with obvious clutter & junk - cardboard boxes, old newspapers, empty containers, etc.
Items that you know are family heirlooms, though, you may want to be careful of.
One thing that you can use to your advantage in this is text messaging. If she has a cell & you have a cell & both have the capability to send/receive picture texts, then you can take cell phone snapshots of items that you aren't sure of, and send it to her with the question Is this okay to get rid of? All she'd have to do is respond yes or no.