Yes, you can add a Filter Step on Zapier to filter posts by their category id. I am not sure if Zapier filters are only available on premium plans though. Have a look at the ’ Configuring which posts the Zap will retrieve’ and ’ Adding a filter step for finer control over posts’ sections of this topic: Use the Zapier Discourse Zap Templates.
If you are on a Zapier premium plan, it will be cheaper to use a Discourse webhook than a Zapier filter to control which posts trigger the Zap. If a Zap is stopped with a filter, it is still counted as a task on Zapier.