Is this frowned upon? Expected? I have never made a presentation for a research proposal before and I'm not sure about what is expected. I was considering using a timeline to illustrate roughly how long the research will take and what the milestones in the research process will be.
The exact expectations for a research proposal will vary by context and audience (e.g., a formal proposal to a government funding agency will require specific rules to be followed) but, in general, any aid you can provide your audience in understanding your planned research is helpful.
It's easy to neglect details like the amounts of time and other resources your research may require. Visual aids can help to make those details clearer. As you suggest, a timeline can clarify the time your research will require. Similarly, a flowchart is often an excellent way to establish not only the order of your project stages (or even if some stages will be concurrent) but to identify outside resources you plan to use, like databases or established protocols. A single figure may also serve to explain detailed methods. That's especially useful if you have a page limit to deal with.
So yes, include helpful figures if you can, as long as they further the goal of explaining what you plan to research and how.