What does "Fresst meinen Sternenstaub, ihr Langweiler!" mean? I tried google translate but nothing showed up.
tl/dr The other answers in this thread are quite correct, but with all the other items on the web in mind the best answer would be "Eat my stardust, suckers!"
And here is why:
Unicorns became cool again with the help of the "Dispicable Me"-movie in 2010. Especially because of this scene. Soon there were memes, clothings, chocolate and toys all over the internet.
With this in mind designers and marketing crews considered what to do next to sell their stuff. Unicorns are magical creatures and when you make them cute and fluffy like in the movies and no noble creatures like in classical fantasy books they become colorful and sparkle. And what else sparkles? Oh yes, stardust, which fits unicorns perfectly.
From there on its just a step from the english mockery eat my dust to "eat my stardust". And then someone added "suckers" because it fits the sentence quite well. You can find this sentence on a lot of items Google.
And then someone in Germany saw this and wanted to sell these items too. He (or she) translated it to "Fresst meinen Sternenstaub, ihr Langweiler!”. Sucker doesn't fit well to Langweiler, but its quite a good sentence and gets the spirit.
It's similar to Bart Simpson's "Eat my shorts".
Obviously it's a colloquial phrase created by some designer who committed to the ongoing "unicorn" trend.
A unicorn stands for freedom, self-fulfillment, emancipation, never giving a damn on what others demand or expect one to do.
The unicorn proceeds making its own way, dispersing a cloud of "stardust" on its way to universe. All the "Langweiler" (slowpokes) can do is inhaling/eating (pejorative: "fressen") the dust while staying in their daily grind.