Firstly, the target Agent's stated requirements, if any, should be followed.
The purpose of a Query Letter is solely to have the Agent ask for the manuscript- it's the time for brevity and clarity, not the opportunity to ramble on about all the characters and story or all the ills that have befallen one.
Our Query Letter will most likely be one of a stack in the Agent's in-tray from total strangers - and unless we've written a darned good focused Query Letter ours will be in the reject tray within a minute or less.
In the final analysis if the memoir is complete and the Query letter route is followed, it all depends upon what is in the Query Letter and memoirs are a very hard sell unless one is a celebrity or has a really truly unique and unusual experience to relate. Tales of overcoming abuse, for example, are legion.
The Agent is obviously looking for a memoir with a target audience and that he thinks he can successfully sell to a publisher - and that will be what he wants to see in the Query letter. The Agent is not going to dwell on the contents of any Query letter if they don't immediately catch and hold his attention.
A novel is deliberately written with interesting characters in manufactured situations designed to thrill and appeal to a wide range of readers and can be doctored with that in mind. A memoir that by definition is based on reality doesn't have the same flexibility and if it's basically interesting only to whoever wrote it plus their friends and family that is going to be pretty obvious fairly quickly.
Good luck. There are many here far more experienced than I in the Memoir department and I'm sure someone else will respond shortly.