Johnson is one of the most common surnames in America, which is precisely what makes it funny as a pet name. Giving a dog a surname as a first name, with no first name attached, is a specific comedic register: "This is Johnson. Just Johnson." It's the pet naming equivalent of an entire bit delivered in four syllables without breaking eye contact.
The Surname-Only Pet Name Tradition
A small but committed population of pet owners names their animals with standalone surnames (Johnson, Henderson, Williams) and the humor comes from the bureaucratic formality applied to an animal. Johnson is more common as a surname than most, which amplifies the joke: the most statistically average American surname on the most statistically average dog. The contrast is the point.
Boris Johnson and Political Timing
Boris Johnson's tenure as UK Prime Minister (2019-2022) gave Johnson a specific politically inflected layer for pet owners who followed British news, and the character traits frequently associated with that public figure (charismatic, slightly chaotic, difficult to contain) are arguably ideal dog characteristics. Whether this reading is intentional varies by owner.
Counter-Reading: Zero Emotional Warmth
Johnson offers no affection in the name itself. It's pure deadpan. Owners who want a name that sounds warm when called may find themselves reaching for something softer within a few weeks. Buddy delivers the warmth if that's what's actually needed. Johnson is for owners who are genuinely committed to the bit and have no interest in the warmth.