The short answer is: yes.
It’s true, these days everyone listens to music on earbuds, and even the cheap external speakers in your tablet or phone are stereo. So it might be tempting to think that with no real mono listening devices, mono compatibility doesn’t matter anymore.
But the thing to remember is, while true “mono” listening may not happen much these days, listening on devices with compromised or non-existent stereo fields is not. For example, those phone and tablet speakers may be “stereo” but there’s no real stereo separation possible from a phone’s tiny speakers. Or if you’re listening to a Bluetooth stereo speaker, the speakers may be stereo, but they’re very close together. These devices may not be “mono” but if you mix to ensure that your track still comes through without any stereo separation, you know it will sound good through these devices with hardly any stereo information.
Plus, checking mono compatibility is still the best way to ensure that there aren’t phase cancellation problems in a stereo mix.