Ugh, that's horrible. Nobody deserves to die young, let alone suddenly. And even not suddenly. It's not just idols, it's every young person.
But this is a reminder that they're only human. Please do not make light of this situation or make jokes about it. The only thing you should say/do is offer condolences, and it doesn't matter if you were a fan or not. That's not the point. It's incredibly disrespectful to joke about death. (Yes, people are being shitty about this, sadly.)
Also, I hope Rina's family, the other Ebichu members and Stardust staff are getting the support they need. Death affects people much more than you'd think, especially when it's a sudden death. You don't get over it quickly...
RIP Matsuno Rina, thank you for everything you've done
.