Having read the thread, I think the poster was asking for a forum in which Latin-rite Catholics who normally attend the NO Masses and are trying to practice their faith and want to learn more without being bogged down in whether Vat II is legit or not. To try to answer the OP’s question, I think a Mainstream Catholic is a Catholic without any additional labels (e.g. Traditional Catholic, Eastern Catholic, etc…). Such a forum could be a place for people who believe/want to believe or understand the teachings of the Church to talk and learn more without politicizing the Church. The Church is neither liberal nor conservative, She is simply the Truth.
This is my rant on what a Mainstream Catholic is, and yes, I consider myself one while believing what the Church teaches. Sidenote: labelling these individuals “Mainstream” contradicts my definition, but whatever.