The mobile theme was deprecated in March, 2020. Follow these directions to make sure your site is prepared.
Originally we created the mobile theme feature as a fall-back when the regular theme did not include a mobile view. Most themes include a mobile view by default now, so the feature is no longer necessary.
It’s very likely that your current theme is already mobile-ready. Your theme documentation will indicate this. If you can’t find it there, you should reach out to the theme developer directly.
If you find that the theme is not mobile-ready, also known as “responsive,” it would be better to find a theme that is responsive than to look for an alternative. With Jetpack, you can choose any one of the themes available in your WordPress.com Themes list. These are all responsive themes.
Responsive themes offer an easy way to set up a website that looks great on any device – that includes your desktop computer, laptop, tablet, or mobile device. In addition to loading properly on any device, the best responsive themes also perform as needed and are easy to navigate on any screen size. Responsive themes also remove the need to have a specific mobile-only theme – the theme will respond to the device viewing the content and resize the content accordingly.