How do I include a CSS library in my custom module?
Fundamentals

To bind a CSS library into your Drupal installation, it should be defined and exposed via a *.libraries.yaml file.

For example (taken from core's classy theme):

book-navigation:
  version: VERSION
  css:
    component:
      css/components/book-navigation.css: {}


defines that there is a library called book-navigation that when included in a page will deliver the css/components/book-navigation.css css file.

Using composer with 3rd-party CSS libraries 

Composer works easily and reliably for PHP dependencies, using the merge-plugin in the way you started working in the OP. But, CSS and Javascript libraries, especially when some preprocessing or building step is required, are a different story.
This is an open and unsolved (to my knowledge and to the date) problem, you can read this issue to get an idea of what problems arise:


https://www.drupal.org/project/drupal/issues/2873160


The main problem is, that when you import the library as a composer dependency, as you did, the library is installed inside the vendor directory wherever that is located. So, you cannot assume much inside your *libraries.yaml file about the file path to the CSS/JS file(s) you want to use.

Alternative (A) without composer: using CDN / externally hosted libraries definitions

For you case, and taken that you want to include this file:

https://raw.githubusercontent.com/geedmo/yamm3/master/yamm/yamm.css


you could define it inside your *libraries.yaml file as an externally hosted library (see #external):

my_module.yamm3:
  remote:  https://raw.githubusercontent.com/geedmo/yamm3/master/yamm/yamm.css
  version: VERSION
  license:
    name: MIT
    url: https://github.com/geedmo/yamm3/blob/master/LICENSE.txt
    gpl-compatible: true
  css:
    theme:
      https://raw.githubusercontent.com/geedmo/yamm3/master/yamm/yamm.css : { type: external }


N.B.: This approach should always deliver the latest yamm.css version.

Alternative (B) without composer: keeping a local copy in your own module's folder

If you place the CSS file inside your module's folder, e.g. css/yamm.css, you will be able to use the following yaml definition in your *.libraries.yaml file in order to be able to use it:

my_module.yamm3:
  version: VERSION
  css:
    theme:
      css/yamm.css : {}


Attaching a library to page(s)

Finally, after you are done defining your library, you can start using it.

This is documented extensively here (https://www.drupal.org/docs/8/creating-custom-modules/adding-stylesheets-css-and-javascript-js-to-a-drupal-8-module) and I would recommend reading that if you haven't already.

Good luck!