Hi! Your Debian packages are built without Multi-Arch support, so it's not possible to install say 64-bit MariaDB in a 32-bit Debian system (for example I want to do it to experiment with TokuDB on my 32-bit server; I use 32-bit because it consumes considerably less memory compared to 64).
I suggest you to make libmariadbclient18 Multi-Arch: same (and place libraries in /usr/lib/i386-linux-gnu or /usr/lib/x86_64-linux_gnu, not just in /usr/lib) (Multi-Arch: same means both 32-bit and 64-bit versions of package can coexist), and mysql-common and mariadb-common Multi-Arch: foreign (this means such package can satisfy the dependency of a package of a different architecture).
That's a rather simple packaging improvement.
More info is on https://wiki.debian.org/Multiarch/HOWTO