Can mom pay off our mortgage?

It could cause problems if Mom needs LTC within the next 5 years with Medicaid footing the bill. Her money needs to be used for her care before applying for Medicaid.

But you plan on paying it back. If me, I would have a contract written up between you.

Yes she can, but it would be considered a gift (check with tax attorney). She would need to file the proper gift tax form with her taxes at year end to account for this, which decrements to overall of 12M she is allowed to legally gift to you over a lifetime.
This form is only for record keeping of her gifts. If it is NOT filed, then the gift tax will be your responsibility to pay. Please google some basic questions on this and also see the irs.gov website for detailed info.
Disclaimer: If she is paying $30K or less, then it will not need documenting, as she is allowed to gift $15K to you and $15K to your husband without documenting.

Now if this is truly some sort of bridge loan, there must also be legal documentation to account for her giving it and you repaying it. Check with attorney after you read irs info.

I know about the Medicaid thing. We went through that with my MIL. We will definitely give back every cent. I'm sure we can get a contract notarized.

Not quite the same thing, but my mom lent money to my kids to help with college tuition. She was pretty savvy about tax ramifications...she paid the colleges directly, rather than give the money to the kids to pay, and she had them both sign promissory notes laying out the amounts and the terms of the loans.
She had enough money plus a long term care policy, so a Medicaid look back was not likely to ever be an issue.
I paid back the loans from my share if the estate, so my sisters weren't shorted anything.