you can find a couple of fixes for the garbled fonts here....
http://freshmeat.net/projects/ymessenger/
I also had a problem when I tried to access my preferences via the yahoo gui. I would get this error....
Gdk-CRITICAL **: file gdkfont.c: line 239 (gdk_font_ref): assertion `font != NULL' failed.
Gdk-CRITICAL **: file gdkfont.c: line 337 (gdk_string_width): assertion `font != NULL' failed.
Gdk-CRITICAL **: file gdkfont.c: line 413 (gdk_text_width_wc): assertion `font != NULL' failed.
Segmentation fault
I finally got this to work by manually editing my preferences file in .ymessenger folder under my home directory. I changed the default helvetica font to be one that is installed on my system . Now the gui works also.