Everyone agrees that the HomePod was a failure. Nobody disputes that Apple's first smart speaker has terrific sound (some of the best of any smart speaker). But its expensive price ($299), limited Siri functionality, and lack of voice controls for non-Apple Music services like Spotify were big turnoffs.
The HomePod mini is Apple's attempt at a redo and, perhaps, its last chance to gain any ground on Amazon and Google in the smart home wars. It's $99 (good!); sound quality is terrific for a speaker this small (awesome!); Siri's smarter three years later (nice!), and it's got new tricks like intercom and the ability to beam music from an iPhone with a tap (fun!). TL;DR: It's everything the bigger HomePod should have been.