No doubt Dawid Malan played an excellent game but Chittagong Kings lost because of their poor batting performance. 121 was never going to be enough score unless they did an out of box bowling, but it did not happen. The top bowlers from Chittagong Kings likes Binura Fernando, Shoriful Islam and Al Islam were unable to take wickets upfront and therefore it was an obvious loss for them.
Now coming to their batting, Chittagong start was a disaster. They lost 5 wickets for only 39 runs and then there wasn't much batting left. Arafat Sunny did try to stay on the wicket and made 27 runs. This is the 2nd consecutive loss for Chittagong Kings.
In T20 cricket, it is definitely foolish to expect that in a match where a total score was 121. Although there are many examples where even in low scoring matches they have won by batting first. However, Fortune Barishal is not an ordinary team and on the other hand, Chittagong Kings team does not have such good bowlers who can restrict the batsmen to short runs.