Compared to characters like Greninja, Shiek, Peach, and even a character like Tink, we have very little AT's to utilize, and the things we do have are very easy to learn and use. The most complex thing I can think of is Arcfire > Nosferatu which isn't reliable at all. Robin is very unique don't get me wrong, but is she difficult to use. No not at all.
The biggest change that I can think of since Smash4 barely came out from me personally is that I use Thoron a lot less and use the other thunders a lot more.
She has a moderate skill floor due to items dropping, and being able to catch them, but because they flash before they run out you don't even really need to know how much your using in battle. You just have to be aware.
She has a very low skill ceiling from what I have seen. The best Robin's know how to space, using the right thunder and that's all Robin is. Spacing, Arcfire, and thunder for the right moment. Yet, there is better characters with more options to space then Robin.
Overall I believe she is not hard to learn or master (if you are in the right mind set).