Can a patch GPS-antenna be mounted in an inclined position?
Short answer: yes.

If you look at the radiation pattern you can observe that the antenna "sees" the sky all around with the same "sensitivity" (ie. is omnidirectional). Horizontal mounting is ideal, because the satellites can be anywhere in the sky.

When such an antenna would be mounted at 90 degrees (extreme tilt), then half of the sky would be shielded by the ground plane, so the receiver could get signal from half of the satellites. Most of the time it should be enough to sustain a fix, so I think a small tilt can be accepted.



Picture from: https://www.semanticscholar.org/paper/Installed-Radiation-Pattern-of-Patch-Antennas%3A-on-a-Gao-Wang/2ffd12945b0588bf76fcd0408cc0d9c1c82737d8