I wonder if the middle finger is censored in the english version(I don't really care, cause I don't watch eng dubs anyway, but still i enjoyed something done by Bulma for a first time). And I'm feeling like this episode is much of an important one for the whole Dragon Ball(and probably DBZ too, 'cause I'm about to watch it after I finish this one).
The way this transformation occurred was definitely convenient, but there's two things. 1: Dragon Ball is not taking itself seriously right now, it's very comedic and lighthearted. 2: There has been foreshadowing since the beginning that something was up with Goku's tail.
This is how you do a scene like this, and even if it was a serious moment that was overly convenient, as long as the foreshadowing is there it isn't that much of an issue at all. Comparing this to a certain scene very late in Yuyu Hakusho,
where Yusuke is reborn as demonkin out of nowhere, making it the biggest asspull ever
, there is a big difference in execution. This was a charming and effective way to do such a scene.