I too would associate "Bouba" with the rounded and "Kiki" with the sharper figure, but to me this is just a relatively universal sense of continuity vs discontinuities:

the local curvature is continuous for "Bouba", every point on the curve has a single tangent, which evolves continuously as you walk along this presented curve.

For Kiki: the local curvature is discontinuous: at the inward or outward points, there are 2 tangents, so the tangent abruptly switches value when passing through this point when walking along the curve.

similarly the "B"s in "Bouba" are voiced plosives, while the "K"s in "Kiki" are unvoiced plosives.

I think people would similarily classify "Titi, Pipi, Touta, Poupa, Kouka" like "Kiki" but "Didi, Douda, Bibi, Gigi, Gouga" like "Bouba".

The vowels are obviously voiced by the vocal chord carrier wave "voice".

So when we pass through the unvoiced (T/P/K) plosives, we notice discontinuity like the discontinuously evolving tangents along the boundary curve of the figure.

Obviously mammals like dogs have a concept of continuity, topology, distance, they might get spooked by "open" stairs, because they can see between the planks, etc.

And when we hear voiced (D,B,G) plosives, the voice while muted still evolves continuously.