How common is it to outsource tedious research tasks to undergrads? How ethical or common is it for grad students to actively hire undergrads into their lab for the sole purpose of outsourcing tedious labour?

Is this frowned upon and is it really any different than a typical undergrad research opportunity.
I am a graduate student, and I often hire undergraduate research assistants.

My primary motivation in hiring undergraduate research assistants is to create opportunities for these students to get a sense of what research is about, gain some experience that can help them get a job or admission to graduate school, and help them figure out what they want to do next.

Sometimes my undergraduate students do work that is tedious, because research can sometimes be tedious. This is especially true when they are inexperienced, because it can be hard to see the bigger picture behind what you're doing. (They may not even have the necessary background to understand the bigger picture, at least not until they've taken some more advanced coursework and read a few dozen research articles.) A good supervisor tries to help students see the big picture, but it's hard.

Supervising undergraduate students is a lot of work for me. I don't do it because it somehow helps me progress in my research by offloading tasks that I really need to get done (if anything, it slows down my research). Usually the work I give to undergrads is work that isn't on the critical path for my own personal research, because I can't trust that it will get done quickly/correctly. And in most cases, by the time my undergrads have learned enough to really be useful to me, they're graduating. 

I hire these students because I remember how meaningful my undergraduate research experience was to me, and I want to pay it forward.

To directly answer your question,


  How ethical or common is it for grad students to actively hire undergrads into their lab for the sole purpose of outsourcing tedious labour?


It's probably much less common than you think, for the reasons described above. It can happen sometimes in the context of a research effort in which someone has to collect a lot of data, and it doesn't require any knowledge or skill to do so. But even then, it's often quicker for the graduate student to do it himself.

When it does happen, I see no ethical problem as long as the nature of the work is disclosed to the undergrad before he/she accepts the job. Compared to many other student jobs (working the phones and calling alumni for donations, for example), being a pipette monkey is actually probably one of the better jobs.