My mom is on HRT and lately she complaint she often got muscle cramp at hand and leg. Could it be HRT the cause of this?

Probably not. Muscle cramps are not usual side effects of hrt. She should see her doc to check for electrolyte imbalance.
Hrt. Depends on hrt. Be sure your mom is on bioidentical natural hormones for the hrt. Do not use premarin, (conjugated estrogens) proprem, or other horse derived hormones or any synthetic that is not bioidentical to human hormones. Also try taking calcium, magnesium, zinc, potassium supplements for leg cramps.