You can just replace str2mat by char and it should work:
for ...
ticklabels = char(ticklabels, char(paramDisplay(paramNum)));
end
If paramDisplay is already a string, you don't need to wrap it into char. You probably don't need the loop either, however I'm puzzled by your loop. The for line implies that that paramDisplay is a 2D array (since you're using size to get the number of columns), but in the body of the loop you're using linear indexing. With linear indexing, you're iterating over the rows first. So it doesn't make sense.
I'm also confused about what paramDisplay is. You use matrix indexing, yet it can be a matrix as that can't be an "array of parameter names". Is it a class?
If paramDisplay were a 1D cell array of strings of varying lengths:
ticklabels = char(paramDisplay{:}) %no need for loop
