I've just had this warning from Google and I can't do nothing about it because the settings are all stock SMF theme
QuoteSearch Console has identified that your site is affected by 3 Mobile Usability issue(s). The following issues were found on your site. We recommend that you fix these issues when possible to enable the best experience and coverage in Google Search.
Top issues
Content wider than screen
Text too small to read
Clickable elements too close together
The 2.1 default theme isn't quite that bad as far as I know, but that's really more of a suggestion to fix rather than a warning or error. If you want, you can edit the theme to improve it, or install a different theme that Google likes better.