My answer would be, it depends. While not a common request, I don't know that it's out of the question. At my elementary, our two full-time custodians were beloved staff members who interacted constantly with the kids, acted as crossing guards and watched over them like grandparents.
If they had wanted shirts and it was a year we had plenty of funds available, I imagine we'd have considered it. It would have added to school spirit and wouldn't have been a large expense.
In middle school (and I think with changing policies) our custodians are a night crew of cleaners with little interaction with the students. I doubt that PTO would have considered it a good idea.
And even if the answer is "no", perhaps you could soften it by offering them a complimentary school t-shirt if you can afford it. These are bound to be some of the lowest paid employees of the school, yet the PTO really needs their help for events.
I was approached by our head janitor about supplying shirts for them to wear as uniforms. He would like to have polo-style shirts with embroidery on them that says our school name and eagle or whatever. What do you all think about this? TIA.