No. In Jainism, kissing your mother’s feet out of respect, devotion, and gratitude is not a grave sin.

The strict Jain view is that the intention matters:

• Pure respect, no lust, no violence, no bad intent → not a grave sin
• If there is attachment, sensual intent, or harm → then it becomes spiritually improper

So the strictest honest answer is: No, not a grave sin.