(I'm not 100% certain this is the correct place to ask this, if not, let me know!)
I've been making Bukkit/Spigot plugins for a good while now, and I've been thinking about making mods at some point, which begs the question: Does having previous experience with plugins/java help with the learning process of modding?
Having prior knowledge in Java is always good, but from the plugin code examples I've seen (I don't actually program plugins), plugins have a very different syntax, and only work server-side, meaning that you don't need to deal with any of the hassle that the siding system brings. This doesn't really matter if you just want to make some simple blocks/items, but it could get a little painful if you try to move beyond that.
I suppose what I'm trying to say is that you should be fine, so long as you look up a few tutorials on modding beforehand.