I don't think this is entirely debian maintainers fault. Package names aren't functions, so the string doesn't necessarily map to the thing you want. Left-pad is the modern analog to what I mean here.
Mariadb probably put in for the package name, tbh I can't be bothered to go find out, but I'd bet anything debian team didn't just arbitrarily make that call for users and ship it. 100% guarantee that.
I know your feeling though, that's the sort of thing that should be printing a warning or something when it happens.
In "Stretch", the default MySQL variant is now MariaDB. The replacement of packages for MySQL 5.5 or 5.6 by the MariaDB 10.1 variant will happen automatically upon upgrade.
$ mysql
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 32
Server version: 10.1.26-MariaDB-0+deb9u1 Debian 9.1
Copyright (c) 2000, 2017, Oracle, MariaDB Corporation Ab and others.
$ mysql --version
mysql Ver 15.1 Distrib 10.1.26-MariaDB, for debian-linux-gnu (i686) using readline 5.2
It is pretty clear to anybody ever run the tool that it is MariaDB.
Mariadb probably put in for the package name, tbh I can't be bothered to go find out, but I'd bet anything debian team didn't just arbitrarily make that call for users and ship it. 100% guarantee that.
I know your feeling though, that's the sort of thing that should be printing a warning or something when it happens.