I certainly disagree that this qualifies as "very clear". The user asked to have mysql installed and the very first item on the list of stuff-to-be-installed is called "default-mysql-server". That certainly sounds like positive confirmation that you are indeed getting what you asked for.

Now, if you keep reading, you will see "mariadb-server-10.1" and stuff on that list, and if you also know two pieces of information (that mariadb is a fork of mysql, and that someone at Debian contemplated the idea of substituting one for the other), then you can see that you may have concluded the wrong thing and you can backtrack from there. Though really, at this point you still have conflicting information and it isn't conclusive.

So, I wouldn't say "very clear" is accurate. More like "possible to decipher if you have all the relevant context and are paying close attention".

As UIs go, this definitely violates the principle of least astonishment.