When I read this one, one of the things I thought was that when she wrote Frankenstein, she wanted to tell a story to the world. When she wrote The Last Man, she was telling a story to herself.
It's a tribute and a eulogy to everyone she loved and lost in her life, but without sharing the acquaintance the book is really not all that accessible. At least, it wasn't to me.
Because I'm also currently reading a biography of Percy Shelley, I actually noticed a lot of the characters and circumstances were based on people she knew. So I definitely see what you mean by it being a tribute to people she loved. It was kind of cool in the beginning to spot the autobiographical references, but I wasn't invested enough to care for the whole 461 pages. It does seem more like a story written for herself than for others.
It's a tribute and a eulogy to everyone she loved and lost in her life, but without sharing the acquaintance the book is really not all that accessible. At least, it wasn't to me.