Since Cypress as a framework currently doesn't support native mobile browsers, your test runs on BrowserStack also don't provide this support. Alternatively, you can set the browser's viewport to the required resolution while running your tests.
Reference information:
- Official Cypress's list of supported browsers: https://docs.cypress.io/guides/guides/launching-browsers#Browsers
- Configure the browser's viewport: https://docs.cypress.io/api/commands/viewport#Syntax
- List of Supported browsers on BrowserStack: https://www.browserstack.com/list-of-browsers-and-platforms/cypress_testing
