There's no easy way to change the font of the timestamp. You can use colouring though with the /timestamp -f timestamp-format-here
command, using control codes within the format just as you'd use them in a message.
Unfortunately mIRC won't store the control codes so you need a simple script to put them back in each time mIRC starts (this code would go in the Remotes
section of the script editor):
on *:start:timestamp -f timestamp-format-here
For example if you wanted the timestamp to always be red and use the [HH:nn:ss] format, you'd use:
on *:start:timestamp -f 04[HH:nn:ss]