The "official" word from Dexcom, at least on the G4 model (which uses the same sensors as the G5) is:
• Avoid inserting the sensor in areas that are likely to be
bumped, pushed or compressed or areas of skin with
scarring, tattoos, or irritation as these are not ideal sites
Dexcom G4 PLATINUM User’s Guide | 25
to measure glucose. Insertion in those areas might affect
sensor performance, and you might miss a low or high
blood glucose value.
I don't know how a tattoo would affect an infusion set. I don't think I'd want to put an infusion set over a tattoo mostly out of risk of messing up the tattoo itself if I ended up with some weird, random reaction to the adhesive or tape out of the blue, or an irritated or infected insertion site, or worse, a case of lipodystrophy, though I know it's rare.
Custom artwork from a quality shop with a good reputation can easily start at $150 per hour or more depending on where you live, and a tattoo takes a minimum of 2 weeks to fully heal. To me, they are a work of art, and an investment in self expression and transformation that I wouldn't be willing to risk having to be repaired. As someone with diabetes, I'm also not willing to get tattoos on my lower legs, as I've already seen that wounds there tend to heal a little slower, so I really mostly have space on my back, and my left arm and left thigh since I sleep on my left and wouldn't want to put a CGM on that side