Feedback – does designing for 2 layouts (mobile plus desktop above 1200px) seems appropriate?

The lead dev in my company suggested designing for mobile and desktop above 1200px. It’s a new way of designing for me so I just wonder does one of you heard about this way before? He suggested that design up to 1200 px will be mobile and above 1200 desktop. When the desktop will be bigger than 1200 (ie. 15 inches screens) the margins on the size increase.