This term “underemployment” can have many meanings, and in answering your question I may not have addressed it in the way you chose.
Certainly, one of the things that are most evident and least controversial is the strong relationship between levels of educational attainment and labour market performance. This is well documented, so your likelihood of gaining employment, your likelihood of gaining secure employment, your likelihood of having good earnings, and even of good social outcomes and good health outcomes, and things like these, are all strongly correlated with your level of educational attainment.
We know there is also a strong correlation with literacy, as shown by a study out within the last very few days, for example. For sure, this ought to be a real concern of Canada. There is no question that individuals who have low levels of literacy and numeracy are very disadvantaged in the Canadian labour market; they will be least likely to find employment, they will be most likely to face unemployment, and their employment prospects will be limited—increasingly so—by their levels of literacy.
This actually has sparked an unfavourable observation about Canada by the OECD, that in terms of adult education, we really don't perform as well as we should for a country that actually has a very strong education system overall.