It's bad because it derails the conversation from the topic that was uncomfortable for the "whatabouter". In many (most?) instances, the whatabout topic still a valid comment on its own merits.
In 2022, some were particularly fond of saying "what about Iraq?" whenever Russia's invasion of Ukraine was criticised. IMO the hope was to derail to "US bad". It's defensible to say that "invading is bad" and "a plague on both the US/UK's & Russia's houses for doing it".