In more recent browsers (everything but IE, essentially), there are some useful APIs for working with URLs that will make this easier:

let url = new URL( MY_PLUGIN_WP_REST_API_CONFIG.baseUrl );

url.searchParams.append( 'per_page', 100 );
url.searchParams.append( 'order', 'asc' );

fetch( url.toString() ).then();

With those methods the per_page and order parameters will be added correctly regardless of whether or not the original URL has a query string. But as I mentioned, these aren't available in IE, although there is a polyfill available here. You're using const and fetch() though, so I'm not sure this is a concern for you.

A more blunt solution that would work in any browser could be to just check if the URL contains a ?, and if it does, use a & character to add your parameters, otherwise use a ?:

var baseUrl = MY_PLUGIN_WP_REST_API_CONFIG.baseUrl;
var params  = 'per_page=100&order=asc'

if ( baseUrl.indexOf( '?' ) > 0 ) {
    var url = baseUrl + '&' + params;
} else {
    var url = baseUrl + '?' + params;
}

fetch( url ).then();

Libraries like axios or jQuery might also have ways to handle adding parameters to a URL that are more sophisticated than fetch, which doesn't have functionality for adding query parameters other than including them in the URL string.

In more recent browsers (everything but IE, essentially), there are some useful APIs for working with URLs that will make this easier:

let url = new URL( MY_PLUGIN_WP_REST_API_CONFIG.baseUrl );

url.searchParams.append( 'per_page', 100 );
url.searchParams.append( 'order', 'asc' );

fetch( url.toString() ).then();

With those methods the per_page and order parameters will be added correctly regardless of whether or not the original URL has a query string. But as I mentioned, these aren't available in IE, although there is a polyfill available here. You're using const and fetch() though, so I'm not sure this is a concern for you.

A more blunt solution that would work in any browser could be to just check if the URL contains a ?, and if it does, use a & character to add your parameters, otherwise use a ?:

var baseUrl = MY_PLUGIN_WP_REST_API_CONFIG.baseUrl;
var params  = 'per_page=100&order=asc'

if ( baseUrl.indexOf( '?' ) > -1 ) {
    var url = baseUrl + '&' + params;
} else {
    var url = baseUrl + '?' + params;
}

fetch( url ).then();

Libraries like axios or jQuery might also have ways to handle adding parameters to a URL that are more sophisticated than fetch, which doesn't have functionality for adding query parameters other than including them in the URL string.