default comments

it is likely:

/wp-includes/theme-compat/comments.php

you can also see this in https://developer.www.ads-software.com/reference/functions/comments_template/ in the source code of comments_template()

Is it considered bad to not include a comment.php file if you are happy with the default?

Is it considered bad to not include a comment.php file

possibly, because you are creating problem for the future:

from the code of comments_template():

else // Backward compat code will be removed in a future release
require( ABSPATH . WPINC . ‘/theme-compat/comments.php’);