Error when trying to mount snapshot LVM (XFS filesystem)

There is no need to mount it for taking a backup with dd. Although dd is not recommended for XFS (see below for why), but if you insist using dd just use the snapshot LV device as the input parameter (if):

dd if=/dev/wd1/mongodbSnap of=/path/backup_of_mongodb_$(date -I).img

UUID is the filesystem's unique identifier which is stored within the filesystem metadata. When you take a snapshot of the LV, also this metadata is identical to the original LV in the snapshot LV and therefore they have the same UUID. However, note what XFS's manual page (locally man xfs or online e.g. here) says about its UUIDs:

  Each XFS filesystem is labeled with a Universal Unique Identifier
  (UUID).  The UUID is stored in every allocation group header  and
  is  used  to  help  distinguish  one XFS filesystem from another,
  therefore you should avoid using dd(1)  or  other  block-by-block
  copying programs to copy XFS filesystems.  If two XFS filesystems
  on the same machine have the same  UUID,  xfsdump(8)  may  become
  confused  when  doing  incremental and resumed dumps.  xfsdump(8)
  and xfsrestore(8)  are  recommended  for  making  copies  of  XFS
  filesystems.

TLDR: It's better to use xfsdump and xfsrestore.

Backing up with xfsdump:

• Install xfsdump. E.g. in Ubuntu these tools can be installed from the xfsdump package:

apt install xfsdump

• Take the dump:

xfsdump -f /path/backup_of_mongodb_$(date -I).xfsdump /dev/wd1/mongodb