Actually, I searched on search bar and there are many flappy bird game made by different kinds of guys. So I thought making a flappy bird game is allowed by everybody.
Viewing post in Flappy Bird - The survival game!
Disclaimer : I am not a lawyer.
Making a clone of Flappy Bird is arguably legal, as long as you only copy the game mechanics I think.
Any artistic property that uniquely identifies the original title, be that the bird's design, the overall color scheme, the title font, etc, or any combination thereof can be enough for copyright protection. A general rule of thumb is :
If you show random people on the street the content, would they identify it as the original (infringed) content? If a large portion of the people would misidentify your game as the original "Flappy Bird", then you're in a bad spot. And your game is not even such an edge case. You straight up copied both the name AND major parts of the design.
That other people are doing the same should not be taken as an excuse, since it wouldn't hold up in court. Not unless it has been done for many years already and there's evidence that the original copyright holder knew of the problem but intentionally didn't address it (I'm not even sure even that would make a substantial difference, please consult a lawyer on this).
As it stands, "many people doing the same" only makes it a more juicy target for a joint lawsuit.