Too much of a good thing?
Too much dual? Nah.
Too much Tomahawk time? Yeah.
If you're gonna instruct for a long time (out of sheer necessity) it's helpful to get a variety of experience.
If I were an employer I'd want to see that you were proficient in basic flying, instrument flying and multi-engine flying.
If you're at a large flight school you should be able to show you're not just another CFI.
You teach all the groundschools.
You instruct in all the airplanes.
Maybe you're even the Asst. Chief CFI.
Employers just want to make sure you've got what it takes to advance within their own company.
If you work for a little Mom & Pop FBO with two airplanes (a C152 and a Tomahawk)...well times are tough and you gotta do what you gotta do...but after about 1000 hours of C152 time I'd probably recommend you start thinking about finding another CFI job with IFR twins and, even better, a charter dept.
Good luck.