As examples I hold up PHP, most PHP applications, MySQL, Matt Wright's script archives (a series of security holes masquerading as useful Perl scripts that were popular a decade ago), and so on.
The correlation is hardly absolute. For instance SQLite is both excellently well designed and also designed for easy install. However it holds often enough to strain coincidence.
My suspicion is that part of it is that something that is designed to be trivial to use tends to have a low barrier to entry to get involved with. This leads to getting contributers with more enthusiasm than skill.