Actually, I would venture that Wilco was as popular as Billy Bragg in the US when the first Mermaid Avenue album came out. They already had two relatively successful albums out.
Wilco headlined at the old 9:30 club on their first tour (I was there, great show), and were headlining venues the size of the new 9:30 club on their second tour. (I was not in the area, so can't say for sure if they headlined at the 9:30 that tour.)
Of course, if your sphere of music revolves around Britpop/modern rock, they probably made very little impact on your musical map. But then again, current bands like OAR, Robert Randolph, and Guster, who all sell out the 9:30 Club, are probably relatively unknown to the typical Billy Bragg fan.