sshfs is using sftp under the hood and the umask for creation new files is handled by the remote sftp-server. You can set umask as an argument to the sftp-server in /etc/ssh/sshd_config on the server, such as
Subsystem sftp /usr/lib/openssh/sftp-server -u 027 # Debian/Ubuntu or
Subsystem sftp /usr/libexec/openssh/sftp-server -u 027 # RHEL/Fedora or
Subsystem sftp /usr/lib/ssh/sftp-server -u 027 # Arch The umask settings and extended ACL are not transferred through the SFTP protocol as implemented by openssh. Also note that there is no "umask on files", but umask is always associated with running process creating the files.
