In a word: no.
Before mendacious "movement" mainstreamers start pointing at Trump's current popularity as a validation of their approach, let's take a closer look at what's going on.
All of us, including the mainstreamers themselves, will I think agree that what the mainstreaming debate is about is public perception. Very well. What then is the perception of Trump?
Trump is no racial nationalist. He has not even been much of a "Republican conservative" before this election - he has contributed to, and socialized with, leading Democrats, he has praised legal immigration, he has previously denounced the idea of "self-deportation" as too tough. He's befriended Negro celebrities on his TV show, and currently has Negro supporters. These things are known to the public. His Republican opponents have publicly pointed out Trump's past political "promiscuity," public love of pop culture has made them familiar with Trump's persona on "Celebrity Apprentice," and the mass media is recently focusing on Negresses supporting Trump (perhaps in a sly fashion to chip away at Trump's support in "implicit Whiteness"). Thus, despite leftist hysteria to the contrary, according to public perception, Trump is merely a boisterous, egocentric businessman and celebrity, at most a "Republican conservative," certainly no racialist or far-right nationalist.
Thus, Trump must be judged according to the public perception of him as some sort of Republican conservative (which is more or less correct, at least as of now if not in the past). Trump cannot be judged according to the standards of racial nationalists, since he is not such and is not perceived by the public as being such.
In the context of "Republican conservatism," what is Trump? Mainstream? Hardly! Mainstreaming in the GOP is best represented by Jeb Bush, whose tepid campaign and centrist platitudes have alienated the base and failed to excite or even interest the general public. Within conservatism, within the GOP, Trump represents a more extreme, populist, "radical" fringe. Indeed, as Trump has moved further "to the right" politically, become more "extreme," as he has peddled in boisterous "speaking his mind" rather than careful mainstreaming, his popularity has increased. Within the context of conservatism, within the context of public perception of Trump as a Republican candidate, his current popularity is a refutation of mainstreaming, and is more supportive of a public who values radical plain-speaking. Now, since America is an oligarchy ruled by special interests, and these interests are strongly opposed to Trump and favor Bush, it is very possible that the 2016 GOP candidate will be Bush or some other unexciting mainstreamer whose boring campaign will result in low turnout of the base and another electoral loss. That outcome is irrelevant to the main thesis here, which is that, given context and perception, the rise of Trump is a clear repudiation of mainstreaming and is more supportive of an assertive radicalism. One can note also the enthusiasm, on the Democrat side, for the left-wing populist Sanders, which mirrors that of right-wing populist Trump in the GOP. The Establishment may eventually get its way, but the populist undercurrents are highly visible nonetheless.
Another point: it may be "illogical," but the importance of public perception means that two different candidates, coming from two different political pedigrees, can promote exactly the same policies but be perceived and received by the public in completely different manners. Thus, it will not serve mainstreaming nationalists to mirror Trump's aracial rightist populism. When a Republican conservative comes out strongly against illegal immigration, that is perceived as radical, cutting edge, populist, and exciting. If the same policies and talking points are expounded by a far-right racialist/nationalist, the perception will be of weakness, mainstreaming compromise, unexciting centrism, and being untrustworthy. It is not only the political positions that matter, but the public perception of how a candidate is moving towards those positions. If a candidate sticks with their principles - that's great. If a candidate moves in a more radical direction - great too. However, if those same positions are taken by a candidate moving towards the center - that's not good at all, that's the "beta/omega male" attitude toward politics - it will disgust and alienate the candidate's base of supporters, make the candidate lose the respect of friend and foe alike, and also most probably fail to win over many of the targeted centrist voters, who may not at all trust this sudden lurch to the center. Indeed, one often sees the opposite, as centrists like Sarkozy and McCain win elections or primaries by moving to the right (which is more successful since rightist voters are more naive and/or desperate and believe that these shifts to the right are real, in contrast to centrist or leftist voters who are more mature and skeptical. Is Trump feinting to the right as well?).