X is pointing fore and aft. Y is side to side. Z is up and down. Why yours seems to display such high vibes in the X axis is somewhat confusing to me. When I mount to the side like that (my favorite method on larger helicopters), I normally get the highest vibes in the Y axis (side to side).
On my Trex 600 I had vibration problems. I made a mount whereby I put the pixhawk on the top plate of one of those "jello ball" mounts with two round 3m tissue tapes. I put small o-rings on the corners of that plate. And attached them to eyelets with the o-rings slightly stretched. Then I put a piece of thin foam insulation between the plate and the frame. And covered the Pixhawk with more of that foam insulation.
In addition to all the above, I also put a heavy aluminum bar between the pixhawk and the plate. The aluminum bar helps dampen vibration. it was fine after that.
Let me see if I have a photo of that installation and I'll edit this post and link to the photo of it.
Here it is without the cover on it. This is lower profile than those jello mounts, just a little stiffer so it doesn't let the pixhawk float around as much as the jello plates. And it worked very well on this helicopter. Also make sure that PVC sheath on the GPS/compass cable is not touching the pixhawk's case. I found that can transfer vibration from the frame to the case of the controller. And make sure you have plenty of relief on the wiring because the wiring is another source of transferring vibration to the Pixhawk. Don't bundle the wiring with a plastic tie - instead let the individual wires "float". And adding mass to the pixhawk helps. You may be able to just see the aluminum bar that is between the pixhawk's case and the plate. That bar's mass helps dampen vibration.