The dependency problems for Matplotlib with LaTex markup
are so notorious that they are already mentioned in the
Matplotlib tutorial [1]:
"On Ubuntu and Gentoo, the base texlive install does not
ship with the type1cm package. You may need to install
some of the extra packages to get all the goodies that
come bundled with other LaTeX distributions."
That note is outdated only insofar as the problem is
no longer with type1cm, a package which does not exist
in recent Debian versions.
Starting with Matplotlib 3.2.1, the critical dependency
is rather on file type1ec.sty [2], which is provided
by package cm-super (or cm-super-minimal).

I see the following possibilities.

Solution 1.

Let python3-matplotlib depend on cm-super-minimal, dvipng,
texlive-extra-utils, texlive-latex-extra.

Solution 2.

As solution 1. Additionally, provide a new package
python3-matplotlib-minimal for basic Matplotlib without
LaTex markup.

Solution 3.

Leave python3-matplotlib without support for LaTeX markup.
Provide a new dependency package python3-matplotlib-latex
that depends on python3-matplotlib, cm-super-minimal, dvipng,
texlive-extra-utils, texlive-latex-extra.

[1] https://matplotlib.org/3.5.0/tutorials/text/usetex.html
[2] https://github.com/matplotlib/matplotlib/issues/16911

control: tags -1 +wontfix

for now i prefer to keep the situation as-is, with extra (rarely used,
in the general sense) packages listed in Recommends/Suggests.