Yes, it absolutely has everything to do with cosmetics. The Federal Assault Weapons Ban (AWB) passed in 1994 banned the sale of assault weapons to American citizens. Since "assault weapon" had no technical meaning, they had to define it. The definition was semi-automatic firearms that shared too many cosmetic features with their fully automatic counterparts. It also banned certain things you could put on your gun because it could start to look too spooo-ooo-oo-ky.
This also included the ban of high capacity magazines. One of the perpetrators of Columbine, Eric Harris, was completely unaffected by the regulation where you could only carry the normal 10-round magazines. He simply brought more of them, 13 magazines in total were found and he fired a total of 96 shots. Then there was the Virginia Tech massacre, which was even worse than Columbine. Cho used 17 magazines and fired 170 rounds. He didn't use any spooky "assault weapons", just two pistols.
In the end, "assault weapons" only accounted for 2% of guns used in the commission of a crime and this was before the ban was put in place. It expired in 2004 and wasn't renewed because it failed to have an impact on gun crime. Derpidy derp.