Looks like your clients constant is not array. You may want to wrap it in formatDataAsArray function. And then formatDataAsObject back (see code below).
Also, as @iiLaw mentions, better assign clientSearch to a separate variable (I'm not sure how these transformers work myself).
So your result transformer code may look like this:
const clients = {{ formatDataAsArray(getClientData.data) }};
const str = {{clientSearch.value.toLowerCase() }}
const result = clients.filter(client => client.client_name.toLowerCase().includes(str));
return formatDataAsObject(result);