Personally I don't have any experience monitoring Informix databases, but Dynatrace should have the ability to monitor them. Whether you can monitor them all depends on the technology you are using and how that technology is calling the database. You'll first want to make sure that the application making the call has deep monitoring enabled. After that as long as the processes making the calls are Java, .Net, PHP, or Node.js and the interaction with the database relies on a supported database framework you should be able to achieve monitoring of an Informix database. I'll place a link below with more info on DB activity monitoring.
Hope this helps!