It should be possible to add a switch to the /drawtext command that disables ClearType, however it may not be as simple as that.
mIRC uses a standard method across all routine to create fonts using the default settings recommended for each font, eg. some fonts will be displayed using ClearType, whereas others will not - Windows decides this based on the font, the display, and so on. Fonts that are designed for ClearType may look far worse without it.
Adding support for the ClearType setting in /drawtext should allow you to /drawfill cleanly around the text (although /drawfill cannot fill in parts of the text that have closed loops - so it may not make sense using it in this context) but the resulting font may not be that readable without using some of the other Windows API
CreateFont() options. Unfortunately it would be far too complicated to add support for all of those options.
In any case, I have added this to my to-do list.