You can't change the orientation of the column labels of the table. However, there are a couple of possible work arounds. The easiest option is to put a new line after each character of the column label so that it looks like this:
L
a
b
e
l
The other option could be useful for a very limited use case. If you absolutely have to have the orientation changed, then there is a messy way to do it that will work for a table that won't be scrolled horizontally, change column widths (you'll have to disable ATTR_ENABLE_COLUMN_SIZING), or be moved. You can use a graph and hide everything but the left or right Y-axis label of the graph, and place it over the table columns. The table can't be scrolled horizontally or have its column widths changed since t
he graph controls that are acting as the column labels won't move as the table is scrolled or column widths are changed. You also won't be able to select a whole column by clicking on the label.
I've attached a uir with a table that shows the different options.
