A lot of the clean code can be attributed to Laurent, the original author of SFML, who kept the philosophy of "simple" true over the years

When I joined the community over a decade ago, the C++ quality was definitely a factor for me, compared to more low-level C alternative libraries.
From all the programming languages I know, C++ has the widest variance of code quality in my opinion. Even PHP and JS have no chance here. A big part of the reason is the number of paradigms that C++ has enabled, and the big changes in usage over its lifetime.
It's not just that there is a lot of bad C++ code, but there is an almost creative variety
of different ways how code can be bad.
We really have everything:
- C with classes, manual memory management and a big pointer mess (LZMA, most gamedev-related libraries)
- over-engineered template hell (several Boost libraries)
- Java in C++ (Ultimate++, many UI frameworks)
- macro hell (wxWidgets)
- antiquated design (standard iostreams)
- over-fashionable design (using template parameter packs and trailing return types just for the sake of modernity)
- wheel reinventors (almost every gamedev library w.r.t. standard containers, linear algebra libraries)
For a long time, truly well written libraries almost seemed like a niche, a gem, to find in C++. There are so many different styles to write C++, it's hard to objectively classify which code is clean. Obviously, the above list is my personal view, and other people might see this differently.
If I needed to summarize what makes libraries good, I would probably mention something along the lines of:
- It has a clear scope, keeps its promises but does not try to solve problems it is not made for.
- There's typically one way to solve a problem, and it's clear which one.
- There's an intuitive relation between types, functions and other symbols in the library, and the number of symbols in total is manageable.
- Abstraction level is high, I don't need to know anything the implementation in order to work with the library.
- Things are named appropriately. For many parts of the library, I don't even need to consult documentation. But if I do, documentation is always helpful.