Although we do not have many details I can tell you the general rules.
There is no gift tax when a distribution is made to a beneficiary of a trust as you have described your nother's situation.
Your mother will have to include in income the amount up to 36K that represents income of the trust for the year of the distribution.
That is, if the trust had 40K of income in the year the 36K was distributed then your nother would incluide all 36K as income and it would be the same type and amount of income as the trust (interest, captial gains, dividends, etc.)
But if the trust had only say 20K of income from the trust in the year of hte distribution then only 20K of income would be included by your mother (and the rest would represent the body of the trust that had already had tax paid in a prior year).
So, the general answer is that your mother will include in her income as much of the 36K that is equal to the total income of trust for that year, but no more than the income of the trust.
You should find out from the executor and the tax practitioner of the trust how much income, and what type, she can expect to have to report so she can set aside the money to pay in when she files her taxes (or to make an estimated payment of her income taxes when she gets the distribution).
I hope this helps for you to know the general rule and that you now can ask the proper questions to help ensure she has enough paid in when she files for the year of distribution.
Please ask if you need clarification.