It's clear from Mamie Smallwood's gushing description of Mr. Watson that she regards him as something of a hero. In fact, she actually says that Watson acts like "our idea of a hero." In this neck of the woods it would seem that appearances count for more than reality. Though Mr. Watson may be a man with a disreputable past, none of that matters to the likes of Mamie Smallwood. In her small world appearances matter. If a man looks like a hero, then he is.
And Mr. Watson looks suitably heroic standing there in the sun wearing a white linen suit and light Panama hat. His heroic image is further burnished by standing next to his beautiful young wife, who's carrying their baby in her arms. The whole scene is one of domestic tranquility, which greatly appeals to Mamie's traditional values.
In common with most people of her time, Mamie's concept of the family unit is of a heterosexual married couple with children, in which the head of the house heroically protects his family from the many and varied dangers of the outside world. And Mr. Watson certainly looks the part of a hero, a man who'll do anything to protect his family.