Hi, I am a graphic designer, I have designed a lot of flyers, brochures, etc.. ,I want to publish them in my portfolio, but on a free cc0 mock up(blank page with shadows, bg) from internet, is it allowed? Designing mock ups is not my job and it is very difficult and maybe a waste of time, cause it does not belong to my work subject, but I want the designs to be presented in a more realistic way for the clients.
I would be more concerned about making sure all your portfolio pieces are your work, and not something taken from a stock site that you're passing off as your own.
And yes, mockups are allowed.
re: "Yes, all pieces are my own works, but I want to present, for example, business cards on black mock ups and on an wooden wallpaper(free cc0), is it allowed? "
Yes, it is 100% allowed.
It is YOUR decision.
My only concern would be whether that style of presentation might make it look like your portfolio pieces are something that you just downloaded off the Internet. You want your portfolio to look like it really represents YOUR work.
So that is something to keep in mind. But presenting your work in the manner you describe is not a rule violation.