There are ways to engage with sexuality which aren't demeaning or dismissive. Personally, I would not say that "Booble" qualifies in this respect. YMMV
"Booble" and the other stuff seems like just harmless fun to me. I'm not saying it's particularly funny, but I don't see how is it demeaning or dismissive.
Is joking about this off-limits because Spears is female? If there was a fan-site dedicated to, e.g., a male bodybuilder; with similar attention given to his biceps muscles as is given to Spears' breasts here; would you think of it as demeaning or dismissive?
Another possibly interesting comparison would be with Black Sabbath. That band is responsible for a lot of groundbreaking and beautiful music, but they're are also associated with "heavy metal", which makes them absurd in most people's eyes AFAIK, sadly tainting the perception of Black Sabbath's music.
On the other hand, can you really blame anyone for making fun of the metal steretypes they let themselves be pigeonholed into? Their reputation isn't even an accident, it's connected with their target audience, giving them their niche. Arguably their music was even influential in creating the "metal" culture.
The takeaway here could be that there are usually different angles from which it is possible to appreciate (pop) culture, and it's stupid to let one aspect taint another, but also that there's no point in pretending an aspect doesn't exist or treating it with excessive respect (i.e., making a fuss about people joking about it).
EDIT: another relevant subject here is whether there should be subjects one can't joke with. I think I would like it if nothing was off-limits to jokes (even though I'm not especially funny). E.g., I'm pretty sure September 11 is sacrosanct in USA in this regard, and there are other subjects joking about which could get many a person quite uncomfortable, like child molestation, rape, murder, suicide, etc.
I myself sometimes get quite uncomfortable when I hear jokes about Holocaust and Jews, but the real issue there is not the jokes, the jokes are just a symptom of societal issues which should make one uncomfortable. I don't know what's the best way to deal with those issues, but I'm sure that making those jokes taboo is precisely what gives them a certain attractiveness and power.
It is possible I am overreacting. However, I think there are is a key difference between this and your example.
A male bodybuilder, specifically, has chosen to flaunt their muscles. There is an element of control and consent to the objectification of those muscles. While pop stars obviously use their image, too, the fixation on breasts feels different to me. It is not something that the artist has invited . It reduces Spears to her physical attributes, rather than engaging with the image she has put out. It places the viewer first, not Spears.
"Booble" and the other stuff seems like just harmless fun to me. I'm not saying it's particularly funny, but I don't see how is it demeaning or dismissive.
Is joking about this off-limits because Spears is female? If there was a fan-site dedicated to, e.g., a male bodybuilder; with similar attention given to his biceps muscles as is given to Spears' breasts here; would you think of it as demeaning or dismissive?