In Jain dharma, this is not usually counted as a gravest sin.

But it is still improper and against modest conduct for a householder, because Jain ethics value maryada, sharam, and restraint. Being naked without necessity is generally not appropriate for lay जीवन. For monks and nuns, nudity is a different spiritual discipline in the Digambara tradition, but that does not apply to ordinary household life.

So the simple answer is:

• Sin? Not a “grave” or worst kind of sin.
• Proper? No, it is not ideal in Jain conduct.
• Best practice: Keep privacy and dress separately to preserve modesty and restraint.

If there was no bad intention, no lust, and only practical necessity, the karmic weight is much less. In Jain view, intention matters greatly.