Yes, kind of, almost.
The field has an orientation and lines of flux. The lines of flux emanate from the magnetic poles. The intensity of flux or magnetic strength is greater near the poles, less at the magnets midspan, and the lines of flux diminish as distance increases.
In the case of the mirror mounts, two phenomena are occurring.
First, like all magnets they are surround by a field of magnetism and defined by the lines of flux.
Second, the magnets have the ability to attract to ferro magnetic materials. In this case, the new mounts for the mirrors. Steel being able to be magnetized is therfore an acceptable mount. Materials such as lead, aluminum, plastic or glass are not ferro magnetic, and will not retain any magnetic properties.
The way an analog indicator operates, in simple terms, the meter movement is based on a variable voltage / current. As these are changed, the electromagnet in the meters movement will vary the meters magnetic strength and reposition the needle.
With strong permanent magnets, or strong electro magnets, they always display a field radiating from the poles. This field, or more correctly, the lines of flux can be bent or have thier shape altered by proximity to other items, or shapes.
Regarding the magnets shape, this can also effect stregth of the field and shape of the flux lines.
I do not run Magic Mirrors. From photos I have seen the mount is fastened to the mirror, and the magnet fastens to the chassis.
If the magnet itself utilizes a steel screw passing through the magnets center of the disc, the added length of the screw can very easily alter the magnets true shape of the field and flux intensity.
For an idea, a round magnet with a length equal to the diameter is often less powerful than a magnet of similar weight, but having a diameter noticably smaller than the length.
So, cutting to the chase and waking you all up. Hopefully the magnets did not appreciably cause errors in the gauges. Enjoy vibration free mirrors with easy removal and reinstall. If you do have gauge issues, consider finding digital gauges with a different function and then display fuel level and coolant temp on the main display.