Many dog owner's chose to get their beloved dog tattooed with a series of numbers on their belly in the event of the dog getting lost. The tattoos are sort of used for the same purpose as a micro chip. With that being said these tattoos don't seem to be very cruel. However, the tattoos are administered the same way as a humans so people may feel that in itself is cruel. But people should know that the tattooing itself doesn't hurt the dog like it does a human because they have more layers skin, but it is still uncomfortable to them. The sound of the tattoo gun is want makes them the most uncomfortable.
With that being said tattooing your pet doesn't sound so bad. However, to tattoo your pet just because you feel it would be "cool" for your pet to sport a tattoo, probably falls somewhere under animal cruelty. Getting a series of numbers tattooed on your pet is a whole lot different that getting an actual tattoo design tattooed on them.