In just the ground lead, yeah, not a great idea. Better: ferrite beads on each of the signals; also, what type TVS are those?
Also, what the...fuck...? Who drew those symbols? 'Cuz antiparallel zeners don't do zener things at all, it's the same as two regular diodes shorting the signal to ground! You can't buy such a device anyway, bidirectional TVSs are antiseries!
Anyway... if they're supposed to be, like, SMAJ15CA or thereabouts, they'll have enough capacitance (couple nF) to be worthwhile in combination with a ferrite bead. So you get some LC filter action going. If not, you may want some explicit capacitance (same magnitude).
Bidirectional TVS are appropriate for RS-232, which is a bipolar signaling standard. Don't use clamp diodes (well.. you could if you had the spare supplies in the system, but who uses +/-15V anymore?).
A ferrite bead in just ground wouldn't be advisable, but running the whole cable through a ferrite bead will help take the edge off common mode currents, for all signals equally. Shielded cable helps even more (but the shield has to be grounded into the circuit ground plane or grounded chassis, on both ends, to work).
Tim