There’s no such thing as a ‘flag ship’ phones, even by Apple standards. It’s based on price points, you said most ‘popular iPhone’, it’s not the iPhone that makes a popular, try the price point in general. Especially when Apple slashed the iPhone 11 by $50 dollars comparable to the 2018 E’XR that was $750.
Anything around £700 onwards is considered a flagship. That’s pretty much common knowledge by now.
Price is massive part in this but if the product wasn’t great then so many wouldn’t buy it. So no it’s not just the most popular iPhone it’s the most popular phone period certainly in the UK. It also sold the most in the US. So clearly the screen quality is not an issue for most people