No — it is not a grave sin.

In Jain view, being naked while changing clothes in a private room with your mother is not a major sin if it is only for necessity, privacy, and without lust or improper intention. For a layperson, this is at most a small fault of modesty, not a grave wrongdoing.

Strict answer:

• Not a grave sin: Yes, it is not a grave sin.
• If done carelessly or without need: it can be a fault.
• If done with bad intention: then it becomes spiritually harmful.

For Jain conduct, the best practice is modesty, restraint, and privacy.