it's the first time that I create gerber files for a pcb printing website. I'm looking to the bottom layer with a gerber view, but it doesn't seem to be mirrored...
it's the same view as in eagle, i.e. from the top. Is this ok or not?
Yes - it should be possible to view all the Gerbers in the stackup at once and have them line up. The PCB house will flip them according to whether they're "top" or "bottom".