The problem with buying cosplay costumes to use as lolita, no matter how lolita they seem, is that they are just that- costumes. Lolita is a fashion that takes a lot of creativity to create a good, pretty look, and i think the general consensus is if you wear a cosplay costume, it takes away the originality that goes into a lolita look- instead, you are just wearing a character, The other issue is most bought cosplays, unless you spend as much money as it would cost to get brand secondhand, are made out of materials normally not suitable for lolita
That being said, I personally believe that people are free to wear whatever they choose, but it needs to be within the rules to be called lolita. The rules are here for a reason, to keep the fashion in line and look distinctive. Without the rules, someone could just throw on a frilly shirt and jeans and call it lolita. This applies to cosplay costumes as well, anyone with a bit of lace on their cosplay dress could just call it lolita without the rules in place.
If you really love a characters dress which looks like it has lolita elements, and i've been in this case as well, i just cosplay it, and don't call it lolita. :>