How can I create a contact list trigger for only those who meet BMI criteria?

Instead of triggers have you explored using Actions, they are a bit more powerful in what you can do with the data.

Yes, I just tried this. It is still not moving eligible participants to the Contact List.

Here is my Action and Embedded Data. Is the issue with the action or with something else? Thank you!

Try to make "BMI" embedded data datatype as "Number". Here is the link to set the correct datatype of the embedded data:
https://www.qualtrics.com/support/survey-platform/survey-module/survey-flow/standard-elements/embedded-data/#EmbeddedDataOptions
Also, try to display the value of BMI on next screen just to check what value it capturing or you can check the same in "Data & Analysis".
Also, under Action tab there is a "Reporting" section which shows the log whether the action is successful or not. If its not successful then you can view the detail log as well to find the error.

BMI is displaying as 0 on the next screen, so I'm guessing it's an issue with the Embedded data. Here is the Survey flow currently. Q8 is the weight in pounds, Q17 is height in feet, and Q18 is height in inches such that someone who is 5'4" would input 5 for Q17 and 4 for Q18.
Under the Reporting section, it says "No Workflows." Does this mean the action is not even firing?

Is the embedded data being set before you are asking Q8, 17, and 18? If so you need to move that down under the Default Question Block.

And yes, your action is most likely not firing. Try setting the BMI condition to 0 to see if that is the issue...just debugging...