...Well, let's take a long list of Honorverse secondary and tertiary characters...
and do a search for 'steward,' looking only for named stewards to RMN officers.
en.wikipedia.org
Joanna Agnelli is personal steward to Aivars Terekhov.
Clarissa Arbuckle was personal steward to Michelle Henke, but was killed in action during the Second Manticore-Haven War. Chris Billingsley replaced her after Henke was released from a Havenite POW camp.
Clorinda Brinkman was personal steward to the tactical officer aboard HMS
Hexapuma during
Storm from the Shadows.
Tatiana Jackson is Hamish Alexander's personal steward, first mentioned in Book 8; I was pretty sure his steward must exist as a named character because he's the RMN character whose domestic arrangements have gotten the second-most attention after Honor.
Alex Maybach is or was Alistair McKeon's personal steward, first mentioned in Book 7.
So...
Uh, it's not strictly true that Honor's the only person who gets a personal captain butler. It's just that most RMN officers don't get the level of focused attention from the narrative that brings their personal life and the question of who's doing their laundry into relevance.
I think "Mary Sue" herself stood out because she was such an
excruciatingly stereotypical specimen of the problem (absurdly young age, absurdly always right, absurdly gets a romance with the charismatic male canon character, absurdly this, absurdly that) that it just burns itself into the memory.
Honor doesn't even begin to reach the level of absurdity as the 'real' Mary Sue, but then, the category is named 'Mary Sues' and not 'Honors' for a reason.