There's no rule about "you have to be a lifestyle lolita to be a true lolita!" and I like it that way.
I can't wear lolita on my job (we have a strict dress code), so it's something I can wear on weekends/vacation only. I can't be a full-time lolita, even if I wanted to.
I think there's nothing wrong with wearing it at cons only, there can be a lot of reasons (and being shy/ fearing that it'd be too "strange" outside of an enviroment like the con's one is a good one); even if it's done "costume-like", as a special dress or to feel different for a day, I think it's ok as long as one thakes time to research, learn the guidelines and what's not appropriate and doesn't go around saying "this is lolita/lolita inspired!" when it's not, but I think these are common sense things.
Hi, my name's Lavinia and I'm from Italy. Feel free to use my real name and please forgive my English mistakes