There are two meta attributes that can prevent a user from scaling your website when viewing it on a mobile device:
WP Accessibility replaces
user-scalable="yes" and adjusts any maximum scale value below 2.0 with a maximum scale of at least 5.0. This allows users to increase their view of portions of your site to see it better, which is significant particularly to low-vision visitors.
Many mobile devices already override these settings, but not all, and they frequently depend on settings within the device.