Replacing a specific pattern of values at level 2 in a list

tl = {{True, False, m12}, {False, m22, m33}, {m32, False, True}};

Replace[tl, Except[True | False] :> z, {2}]

(* {{True, False, z}, {False, z, z}, {z, False, True}} *)

One rather literal (string-based) interpretation of your question:

tl = {{True, False, m12}, {False, m22, m33}, {m32, False, True}};

tl /. s_ /; StringTake[ToString[s], 1] == "m" :> X

{{True, False, X}, {False, X, X}, {X, False, True}}

In this way the True and False conditions are analyzed

tl = {{True, False, m12}, {False, m22, m33}, {m32, False, True}}; 
Map[If[ToString[#1] == "True" || ToString[#1] == "False", #1, 
   x] & , tl, {2}]

$\left( \begin{array}{ccc} \text{True} & \text{False} & x \\ \text{False} & x & x \\ x & \text{False} & \text{True} \\ \end{array} \right)$