You can force mongo to ignore the certificate hostname mismatch with the allowInvalidHostnames configuration option
net.ssl.allowInvalidHostnames
Type: boolean
Default: False
New in version 3.0.
When net.ssl.allowInvalidHostnames is true, MongoDB disables the validation of the hostnames in TLS/SSL certificates, allowing mongod to connect to MongoDB instances if the hostname their certificates do not match the specified hostname.
Changed in version 3.0: Most MongoDB distributions now include support for TLS/SSL. See Configure mongod and mongos for TLS/SSL and TLS/SSL Configuration for Clients for more information about TLS/SSL and MongoDB.