It is a very carefully defined legal term used to define pension pots where the total capital sum is under £30,000.
There is absolutely nothing personal about the letter. The person who sent it does not know whether your mother is a multi millionaire or on pension credit.
Every profession has its special language and the legal and financial professions, in particular, do. Most of these words have very clear accepted definitions, for very obvious reasons.
Put the term 'trivial commutation' into google and see the huge number of government, tax and legal sites that come up.
It is not unusual for ordinary words to have very specific meanings, and very different meanings when used by one profession or another.
Just remember professional letters like this are not personal, and the sender knows nothing about the recipient.