Prior to the invention of iMessage - (iOS 6?) - all iPhone texts were this colour of green. It was the same no matter who you texted.
This means Apple never intentionally designed the green bubbles to be more difficult to read.
https://www.gottabemobile.com/how-to-tell-if-youre-sending-a...
White text on green is significantly different - the older green bubbles with black text are noticeably clearer IMO, even if I also find it unlikely it was done deliberately to make SMS look bad. That said, Apple will certainly be aware of the cultural thing of the green vs blue bubbles given major news outlets have written on it (in the US anyway), so...
https://www.wsj.com/articles/why-apples-imessage-is-winning-...