Depends on you circumstances, expectations of 'making a living', area you are in, expertise and lots of other factors I would say?
For example if you were paying top wack rent for somewhere and offering part livery then the answer is probably no, but if you own somewhere wth little or no mortgage, can offer full livery, find good staff and also offer lessons etc so that you can charge top rates for the livery, then the answer is probably a yes.
I imagine you must be able to otherwise no one would open or run livery yards!
If you have say 30 horses and ponies on DIY at £100 a month that would bring in £3000 then you might do extras like feeding them in the mornings and turning them out/bringing them in. Looking after them when the owners go on holiday etc.
You could also buy hay and bedding in bulk and sell on to the liveries with a mark up.
Judging by the amount of livery yards in my area, normally farms who have diversified, run by complete numpties ..... then I think the answer must be yes.
SO1 has some very good points regarding lots of DIYs. I would like to add, or subtract in this case the cost of yard, fence, stable and field maintenance. If you have all the farm machinery to do it yourself then fine, but if not then you need to take into account the cost of contractors to do fencing, harrowing, rolling, fertilising etc. Insurance is paramount when you are responsible for other peoples horses. IMO, Public Liability Insurance is insufficient to cover you and I would recommend getting yourself 'well covered'. Also take into account the rising cost of fuel and utilities.