df.dropNulls can be hard to find

[koperagen]

dropNulls follows pandas convention, similar to dropNa
In stdlib there's list.filterNotNull, and this is what Kotlin developers expect more.
I suggest to add

df.filterIsInstance<Type> { col and col } 
df.filterNotNull { col and col }

[Jolanrensen]

Though we can add them, it would be two functions to maintain.
It might also be an option to add a permanent

@Deprecated("filterNotNull is called dropNulls in DataFrame", ReplaceWith("this.dropNulls(cols)"), WARNING)
fun DataFrame<T>.filterNotNull(cols)