I feel like I write this out every month or two…
The Eleven Supernova are an out of universe term. It was originally the title of a chapter and the fandom latched onto it as a way to shorthand the incredible number of new characters that were dumped on our lap during that chapter. But no one inside the story call them Supernova. We can use it in discussion, but keep in mind that it's never uttered by any characters.
Super Rookie is the proper term for what they all were. A Super Rookie is any pirate who, on their first voyage, managed to reach 100m beri as their bounty (I believe, before they hit Saobody). It seems that the distinction will last even after they can be considered a rookie, which is to say, after a number of years has passed since their first voyage. In addition to the Eleven Supernova, we are also aware of the following Super Rookies of various years: Cavendish, Bartolomeo, Caribou and Coribou.
The Worst Generation is comprised of the Eleven Supernova, and Blackbeard. They are called the Worst Generation because of all the chaos they have caused in the last two years, as opposed to the relative peace that existed during Whitebeard's era. Luffy, because of how wrapped up he was in the events around Whitebeard's death, including the breakout of Impel Down (and thus ALL the prisoners who proceeded to cause trouble), invading Marinford, and then re-invading Marinford after the war ended with his bell-ringing (which a LOT of people saw as him declaring a new era of piracy), is counted towards the chaos of the Worst Generation, even though most of his exploits happened right before or at the same time as the "official start" of it.
If you were to ask Brown Beard to explain it, he'd likely say that, "The Worst Generation, are the eleven Super Rookies who met together on Saobody, and the pirate Blackbeard. After Whitebeard's death, these twelve pirates caused incredible chaos in the world, and are seen as the central figures of the new era." Luffy invading Marinford after the war with a crew of two, including a former Shichibukai and Dark King Rayleigh to hold a short memorial service for Ace and Whitebeard is VERY notable. Even if he disappeared for two years after that, Luffy "rang in" the new era of piracy.
Keep in mind also that bounty is not the same as notability. Killer had a higher bounty because the WG considered him a greater threat, but Luffy was involved in MANY more outrageous and notable events, even if the WG played some of them down in the papers. He just didn't crucify enemy crews and blow up cities that pissed him off.