Your mom will 100% kill you if you dare to say that.
To me, this sounds like you're saying, "My mom is rather old," or "My mom is relatively old."
"老的是我母亲" (or "妈[妈]", which is more like "mom") would often be sufficient. The comparison can be implied in Chinese.
As EthwanChenobi pointed out, the sentence does not sound appropriate to the readers, including me and him as the proficient Chinese linguist. How about these sentences:
You don't have to include "的“ in this case. 我妈妈 alone is fine.
Agreed, 比较 means “quite” or “rather”, not “more”.