Currently, the Sheets integration uses Jira Cloud's API endpoints to retrieve data.
When retrieving values from text fields, a string will be retrieved since that's the format returned by Jira's API endpoint.
That's true even if the text field contains only number characters.
As consequence, Sheets format the values as text on their side, and you won't be able to execute math functions towards those values, such as SUM.
Provide an option to import values from text fields as number in case they contain number only characters.