It's not unusual to include some professional work in an M.Arch application portfolio. If you feel that the work illustrates some ability or experience that isn't fully demonstrated in your academic work then it may make sense to include it.
If this is going to be a single page in your portfolio consider limiting the content of the page to just one project.
Make sure to note your role in the project (is it solely your work? were you a designer on the project? a team member on the CD set?)

i put in 2 built projects, where i was more or less the sole designer, for march admission portfolio. they were not mediocre and not an example just to show that i worked somewhere. if they were i would just let the cv tell that story for me.

for me the rule is don't put anything in portfolio i would not look to do more of in the future, built or unbuilt. this means most of the work i was responsible for in older jobs will never be shown to anyone.

i did however have a set of DD dwgs ready from those less than interesting projects cuz while they were design duds they were also technically quite good. when i was looking for work after grad school they were a good thing to have.