In general, I would answer a huge
no to this ("Should the tulpa phenomenon be 'more mainstream'?").
I'm not against community growth, or the spreading of knowledge and useful information, however this can be something that is difficult to bring about in a conducive manner.
With the perspective that a lot of people, some uninformed, others immature, view tulpas from, tulpa communities could easily take a turn for the worst (some could say they've already taken
one many turns in this direction. Maybe after 4 turns we'll end up back where we started) if such a thing happens (albeit slowly over time).
I saw a tumblr post that was getting a few notes that was essentially "Lifehack: Bored or lonely?? Make a tulpa!!!" which made me cringe. If the tulpa phenomenon spreads, I hope it's not in that fashion.
This is a great example of a bad way for tulpas to spread, and there are many others. People using them as excuses for their actions, people using them to brag or act superior to others (or attention-whore), people viewing them as role-playing, and in general, just the widespread misinterpretation of many things.
Unfortunately, it could be the case that all it takes is one small event (A post to the frontpage of Reddit, or a tweet from someone popular) to severely popularize aspects of the tulpa phenomenon, and depending on how such an event would play out, it could cause some long-term problems, especially for the larger communities.
I think it's a good plan to (try to) keep tulpas where they already are, and not force them into other communities, where the conditions would be very suboptimal.