You can limit which posts or topics will trigger the webhook on Discourse in the webhook’s Triggered Categories, Triggered Tags, and Triggered Groups settings. Creating a tag group that has tags that can only be seen or used by staff members could be a good way of limiting the webhook to specific topics.
You can also add a filter step on Zapier to only continue if the topic meets some condition that you specify. There is an example of how to set this up for a user webhook in the ‘Catch the Discourse User Created Event’ section of Create a Salesforce lead via Zapier when a user signs up on Discourse. Setting this up a filter for a topic or a post on Zapier would be a similar process.
My understanding is that if a Zap is allowed to progress as far as its filter step, that event will be counted as a task on Zapier. It would be best to filter out as much as possible on Discourse before sending the request on to Zapier. Filtering by categories, tags, and groups is the best way to achieve this.