Yeah, especially since this plastic here is PLA and not ABS, and also the yellowing apparently comes from the brominated flame retardant added to it.
A lot of the yellowing of vintage 1980s era Macs comes from their spending their early life installed in smoking-allowed workplaces.
That's okay though; in that case it can just be cleaned off. Better, the residue probably protects it from UV damage, so the plastic underneath is probably not in terrible shape.