|A bodybuilding athlete with smooth, dark tanner.|
Tanning is an important part of bodybuilding. The reason why bodybuilding athletes are very dark is to accentuate the muscle under the bright stage lights. A competitor who is lighter skin toned, but very cut and muscular, will not look as shredded as a competitor who is dark.
A competitor should be smooth, and look like a very, very dark version of themselves. Judges will take note of your skin presentation and if you're too light it will go against your score. That includes if you are blotchy or striped.