I can see a 50hz flicker
No you don't, since
output of diode brige
I just tried it myself and can't see any flicker whatsoever.
But if I just connect a single rectifying diode the flicker is pretty terrible.
Therefore either you're diode bridge is damaged or you have hooked it up wrong.
P.S.: A filter cap is in this case rather problematic since it
raises the voltage by the factor of sqrt(2), like every bridge rectifier with an filter cap. The LED-stripe is designed for a nominal voltage of 12V - about 15V might be too high for the LED-stripe (long term seen).
The so called Average rectified value of a sinusoidal voltage however is 0.9U
RMS. Meaning it is no problem to hook up 12VAC bridge rectified directly to a 12VDC load.