When something is sashless, it means it doesn't have a sash, which is a strip of cloth that goes around the waist or over the shoulder.
Being sashless is neutral, it just means something doesn't have a sash.
Examples:
The dress is sashless, it doesn't have a belt.
I prefer sashless robes because they are more comfortable.
She chose the sashless option for her dress.
The sashless coat looked plain without the belt.
The sashless outfit felt incomplete without the ribbon.
The sashless gown was simple and elegant.