First thing's first. I'm going to say mapping notes to vocals or the lead is often strongly discouraged and should be avoided. There's nothing technically wrong with it, but with following sounds for mania (what you should be doing, might I add), it can get a little cluttered, particularly for 4K and following vocals is the first thing to be dropped.
Going off of that, unlike standard, there are no filler notes in mania. That means that, if there aren't any sounds about, there shouldn't be anything there. Vocals aren't really considered as sounds for this as they're not as clear-cut as something like percussion. If there are notes where there aren't sounds, then those are ghost notes.
Avoid "anti-ghosting" as well, which is not mapping something that should be. For example, something like a snare sound shouldn't be missed. It's more often than not a problem for easier difficulties, as you can't really map many things that appear on 1/4s at high BPMs. The easy solution is to "cover the sound" with a long note (LN). A good place where people use this is in drum fills.
The thing to come after this is consistency, which is essentially mapping the same sound with exactly the same number of notes, well, consistently. A common thing would be to map bass kicks for the harder difficulties as doubles, and this would be the same throughout the rest of the map. Occasionally this can be broken, but there needs to be a very good reason as to why.
"Variety is the spice of life", as they say. Remains true for this too, you know. It's not wrong to be repetitive, but being varied (while being consistent) can make a map interesting.
Another important factor is playability. Essentially, it's trying to ensure that the patterns you map are somewhat comfortable. Of course, you can go against this, but you're going to need a reason to the tons of modders who will point it out. It's important to playtest your map often and think about the players it will be aimed for. (The most common way most people do this is through spreading the notes out across all of the columns evenly.)
Lastly, progression. Easier difficulties should essentially be a more basic version of harder difficulties, rather than a different entity altogether. Simply, there shouldn't be anything in an Easy or Normal that would be absent in a Hard or an Insane, for example. The other thing is, there can't really be too much of a difference between difficulties, as the jump would be unreasonable to players trying to progress onto the harder difficulties.
If anything, practice will improve your mapping skills, as it does anyone. But knowing about these will certainly help, as it has for me.
Of course, if there's anything here that you don't understand, feel free to PM me somewhere.