How do graduate admissions committees view taking a lighter courseload in your final year of undergrad?
It's normal for seniors to take a lighter course load in order to work on their senior research project. Most universities have a "senior project" faux course at the 400- level to indicate this. Some do not.

As far as how grad admissions will view this. As long as you can show that you weren't slacking off your senior year -- perhaps by showing your senior project results in your application dossier, or alternately asking your research supervisor to write for you -- you should be ok. 

If you do feel burnt out, I would take a gap year -- either between junior and senior years -- or right after college. Grad school is a long haul.