I'm writing a report talking about how a certain technique in my field has become 'more mainstream', but that phrase looks rather wrong. Is it a valid thing to say? Can something become "more mainstream" or are things either mainstream, or not? If it isn't a valid thing to say, how else should I phrase it?
The imagery that "mainstream" brings to mind is that of a river with a gradation of currents - the main/fastest one in the middle, and then slowing down as you move toward either side. The closer you are to the middle, the more you're in the main stream.
The point is that whether something is mainstream is not a binary (either/or) question. Thus, it is entirely correct to use modifiers like more and less with mainstream.
(Personally, I find nothing wrong with "more perfect" or "most unique" or "very pregnant", all of which I've seen denounced as redundant and/or contradictory.)
Yes, it’s perfectly acceptable!
…and widely used by good writers, in professionally edited contexts. Searching the NYT, for instance, gives 18 hits for ‘more mainstream’ in the last month alone:
Writing for a more mainstream publication with a larger circulation might have given Mr. Barrett's work more exposure… — New York Times, 25 Feb 2011
Digressing a little: despite the ‘principled’ arguments some make against “more perfect”, “more unique”, etc., all of these can make perfect sense in the right contexts. Very few qualities (if any) are so absolute that there aren’t situations where it makes sense to talk about things possessing more or less of them. I defy anyone to read the beginning of the US constitution,
We the people of the United States, in order to form a more perfect union, …
and argue that it’s bad writing!