I'm not into loli, but i do think that the there is a tendency with any cute character for the writer to hold back and be a little bit vanilla, perhaps it's the idea of this type of character being inexperienced and almost too sweet to be into anything more "flavourful" as if somehow the cute aspect and the kinky aspect are mutually exclusive. I think that in a lot of ways a character that is designed around a particular fetish strongly will be a "mono fetish" character, perhaps that is because they are designed solely for that rather than as whole characters, i mean really who among us can say they are only into one fetish? why not mix it up, a cutie who is a solid dom or sub would be good, even if they are inexperienced, hell it would be more interesting if they were inexperienced.
No i don't think lolicon's are particularly vanilla, just the writers.