You have to return the car as clean as you received it at the beginning of the rental.
Within the limits of normal use, you are not required to clean the vehicle. However, a quick clean is recommended. It makes all the difference to owners and is reflected in their reviews.
If the vehicle is very dirty on the outside (mud, dirt on the bodywork due to driving on unsurfaced roads, etc.) or the inside (heavy mud stains on the mats, sand, crumbs, etc.), you need to clean it. If the vehicle is unreasonably dirty when you return it, you will have to pay the owner a compensation from €15 to €45 (25€ to 75€ in Germany) for cleaning the interior and for washing the exterior. For any soiling which requires work by a professional (stains on the seats, extremely dirty interior, etc.), you will have to pay for the associated cleaning costs.