I wouldn’t say he’s the hardest to learn, BUT he’s definitely more execution heavy than most of the other cast.
All you need to do is mess up one IAT or TK gale, and you will soar through the skies for 70 frames while you whiffed a button so you have no options and landing recovery, most likely eating 200-300 or more damage as a result.
Another aspect of difficulty comes with getting good use of your Yoga Drills as Dhalsim. It’s very easy to space yourself wrong, causing drill to whiff or hit too high. You need to be quite on point in knowing your ranges to get good use of that.
I wouldn’t recommend Dhalsim for a beginner as a starting character. His playstyle is a bit unorthodox. However, he will teach you how to antiair, when to press your advantage and when to chill pretty well.
You will also learn footsies from a bit of a skewed point of view. So play Ryu, Cammy, Birdie for a while and then consider again when you have a better understanding on how the game works.