iirc Kid Buu was the form where he was "purged" of all the "imperfections" he previously consumed. Could a similar concept be applied here?

In this case I don't believe so. Pliabuu's never had to absorb anybody (canonically), so without anybody to purge, she doesn't have a Kid Buu equivalent. Within the context of the game it probably makes more sense...or it doesn't and it's just something the devs are rolling with to give majin a transformation. In order to match up with that using the roleplay I've invented with Pliabuu, her transformation is not into Kid Buu as much as Kid Pliabuu.

well personally I was focusing more on the "imperfections" the transformation would be removing that her regular body would have. (inside and out)

The only noteworthy imperfection Pliabuu has (personality and power level notwithstanding) is her inability to sense ki as an adult, but purging that wouldn't be enough to trigger a transformation. But she's also not very fleshed out, so I might come up with something else if I'm so inclined. Xenoverse 2 might trigger some lost inspiration.