Informix has never been good in developing tools.
ISA was really a big thing at Informix!!
There is Server Studio (AGS) that can connect. First two modules are for free.
Or you could use simple sql, like most informix dba's.
I created my own monitoring database. I put a few scripts in the crontab that fill performance stats.
With Office Connect, I can show nice graphics for the managers...
Finally, IBM committed in tools. They are going to implement the tools they currently use for DB2 for Informix. Isn't that big!