Configure the below Enable Rule for the required ribbon buttons to show Only in Mobile/ Tablets and not in CRM Web.

Use the below function,

function ShowOrHideDeviceButtons() {
 var showOrHideFlag = false;
 // Form Factor = 2 - Tablet
 // Form Factor = 3 - Phone
 if (Xrm.Page.context.client.getClient() == "Mobile" && (Xrm.Page.context.client.getFormFactor() == 2 || Xrm.Page.context.client.getFormFactor() == 3)) {
 // Add code that should only run in CRM for phones here
 showOrHideFlag = true;
 }
 return showOrHideFlag;
}

Output:

Dynamics 365 for phones App View: Highlighted ribbon buttons can be seen only in Mobile/ Tablet.

CRM Web View: Buttons are not displayed in CRM Web View.

Hope you learned a new thing today :):):)