Disregarding costs aspects, diamonds makes a good noble cathode, where I am assuming there is an intended electrochemical use for the coated object (else, this question is on aesthetics).
Generally, for the anode and cathode in the special case of an electrochemical cell, it is known that one can greatly accelerate galvanic based corrosion reaction, for example, other things held constant, by having a small area anode relative to a much larger area cathode.
Here is a supporting answer provide on Research Gate, to quote:
The influence of cathode/anode area ratio is a key factor to the rate of galvanic corrosion in seawater. It can be explained that the anode current is always equal to the cathode current when the galvanic corrosion occurs, and anode area is smaller, the anode current density is greater. That is, the corrosion rate of anode metal is greater. Increasing Sc/Sa accelerated galvanic corrosion rate of high potential difference coupling, the fitting curves of the galvanic current by experiments will show the parabola shape. When anode density is greater than corrosion rate will be higher. Galvanic corrosion rate linear growth with the Sc/Sa in seawater indicates, the galvanic potential shift and driving voltage decreases.
However, if we are talking about battery technology, we are in more complex arena of surface modifications. To quote a Science Direct source:
Electrode surface modifications generally prolong the lifetime and improve the safety of Li–ion batteries. Stabilization of electrode surfaces and the interphases reduces the extent of side reactions, thus suppressing impedance growth, capacity fade, and the release of significant heat and gases which cause safety risk of thermal runaway. Although significant progress has been achieved, particularly during the past decade, the field of electrochemical and chemical surface modification of battery electrodes is still wide open for further exploration. Novel synthetic approaches for electrode surface functionalization and preparation of thin-layer coatings are desired. However, the processing and manufacturing approach of electrode coatings must be facile and scalable at industrial level, as the main bottleneck for the large-scale implementation of Li–ion batteries today are the high cost of cathode materials and processing.4,6 Another practical issue is the stability of the surface modification, for example, the coating, which tends to degrade over time and cycling. Degradation is typically accelerated under harsh chemical and electrochemical operating conditions (e.g., high or low temperature, high voltage, and fast cycling rate). Further fundamental understandings of operational principles of the interphases, the synergetic interplay between their components, and how they may be modified for improved long-term cycling are necessary.
So, some knowledge of the intended purpose of the application for the diamond electrodes would appear to be important as to which is actually better as, for example, diamond-like carbon (an amorphous form of carbon) may not be suitable due to stability/degradation concerns for the targeted application.