A flop is something or someone that is unsuccessful almost right out of the door. Cascada has had her share of success numerous times. So calling her a flop would be using the term incorrectly. She's not as big as other celebrity singers a la Madonna, Britney Spears, Lady Gaga, and so on, but she's done well for herself at least.
I think the biggest thing she will be known for though is her song "Every Time We Touch" even though she has had other hit songs.