Strictly speaking in Jain Dharma: yes, it is improper, and being naked with one’s mother is not considered a right or auspicious conduct.

But it is not described as a “grave sin” in the same way as हिंसा, theft, deceit, or intense indulgence. In Jain view, the main issue is:

• loss of modesty and restraint
• indulgence in body-consciousness
• possible increase of attachment, carelessness, and अशुभ भाव

So the strict Jain answer is:

• Changing clothes privately with your mother: not a sin by itself
• Being naked in a private room: improper and to be avoided
• It becomes spiritually harmful if it is done carelessly, with attachment, shameful excitement, or disregard for संयम

In Jain ethics, purity of conduct, modesty, and self-restraint are important. So the best practice is to avoid nudity and maintain proper decorum, even in private.

Final strict answer:

• Improper? Yes.
• A grave sin? Not usually classified as a grave sin, but it is spiritually unfavorable and should be avoided.