running a program through ssh fails with "Error opening terminal: unknown."
A plain ssh command like that does not have a tty (terminal). Use the -t option to force ssh to open the terminal on its way in.
From the manual:
-tForce pseudo-tty allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g., when implementing menu services. Multiple
-toptions force tty allocation, even if ssh has no local tty.
So this would work (better):
ssh -t servername ~/htopmem.sh