Interesting idea. Reality is the number of people who know the correct answer here is going to be very limited, so I don't think you have to worry about it.
Maggots have their preferences in what they eat, besides just dead, rotting flesh. And I can't remember, but I think tattoos are down on the list of preferred flesh, but I'm not sure. You're also looking at some significant time, probably in the range of two or more weeks. You'd do better as Medievalist suggests and cut the tattoo off.
Adding maggots in a more appropriate fashion would cause some confusion as to time of death, but I'm not sure it would be especially significant. There are some other parameters that a pathologist looks at as well.
But who's going to know this, other then some attorneys and pathologists?
Have your killer slice the tattoo immediately after death, while you still have some blood seepage to go into the cuts, and add maggots. I'm not sure anyone knows for sure exactly what the results would be.
Best of luck,
Jim Clark-Dawe