That photo is from a film about neanderthals. The phrase "unga bunga" has its earliest known usage in a Bugs Bunny short mocking Aborigines, but it's a generic enough phrase that I'm not sure you can write off the entire phrase as racist against Aborigines: any nonsense word could be used in its place and I'm not sure anyone creating or sharing the meme has actually watched that Bugs Bunny clip from 1950. It's just a nonsense phrase used to indicate low intelligence or nonsense. Given that the photo is of a neanderthal, I think anyone seeing the meme will understand that it's supposed to mean "a caveman would be confused by this" and not about any particular group of modern or indigenous humans. Most usages of the meme imply that the poster themselves is the confused one, so I don't think a racist would find it very funny to post something like that.

https://knowyourmeme.com/memes/confused-unga-bunga

Given that the words "unga" and "bunga" have existed in print since at least 1700 AD, I'm not sure we can point to that one Bugs cartoon as the definitive and only definition of that particular nonsense phrase. https://books.google.com/ngrams/graph?content=unga%2C+bunga&year_start=1500&year_end=2019&corpus=en-2019&smoothing=3