Moschops are reliable and low effort but fairly slow, especially if you don't or can't boost resource respawn rate. With a pen you either block resources or make it massive, without one you'll find yourself riding the moschops back to where you want it gathering fairly often. Absolutely get one, but don't rely on it as your sole fiber gatherer.
On Abberration, ravagers are actually able to gather small amounts of fiber and mushrooms from those blue glowy mushrooms in the biolum region. While you don’t get much fiber, it’s good in the early game if you need a small amount.