With a couple of tweaks, your code works as you want it to:
t = datetime(2006, 01, 01):calmonths(1):datetime(2100, 12, 01);
x = 1:100;
y = 1:100;
for i = 1:12
subplot(6,2,i)
plot(x*i,y)
m = month(t(i),'name'); % Isolate Month Name
title(sprintf('It is %s', m{:})) % Use ‘sprintf’
end