In the long run, TBDT did mostly trouble for her, Because of that, people have even more reason to take her seriously,and those who think she's a sell out from the start are like " see, we are right, Avril is just like Britney Spears ".
Even her biggest hit "Girlfriend" is not good for her in the long run, why? Because it's her most known song, It's the song that made people think Avril is a bratty, immature boyfriend stealing sell out, and her serious hit " I'm With You" and " My Happy Ending " have been put aside, because of that song, people have forgot she have a soft, serious side.
Can you imagine how huge her fanbase today if they have not left her in 2007? If she continue with her "legit" anti hollywood image, Avril is filling huge arenas at the moment.
Anyways can you explain how did TBDT save her from irrelvence? Look at the UMS era, her career, her fanbase is very and going strong,when TBDT came out, she lost ALMOST all of her fanbase.
I don't hate TBDT album, I love that album and image, but my point is there's ton of reason why Avril should not do it again, and there's literally hundred reason why it is viewed as her worst album ever, and most of their reason is not even music related.
Like what I said, there's improvement in that album, specially in lyrics department, Yes even her fun songs are witty and creative.
Regarding your question about UMS, Because the image of UMS is her most legit and mature, the angst made her different from those other pop stars who seem embraced the celebrity life style, while the UMS Avril seems to be against it, thus making her more appealing to anti establishment crowd.