I don't dispute the baptism entry - indeed, to all intents and purposes the entry reads as if she was lawful issue of a John Robinson and wife Jane (Simper).
However, it is you yourself that says she was the illegitimate daughter of Jane Simper - how do you know this?
Did John Robinson and Jane Simper every marry? If not, I can only re-iterate that regardless of how the name at baptism is worded if Mary Ann was indeed illegitimate she wouldn't have been known as Robinson but as Simper.
There is nothing wrong with the dates here - the Mary Ann Simper buried at Pakenham 15/5/1841 was aged 26 - the baptism was 18/9/1814 and if born shortly before then, then Mary Ann wouldn't have been 27 until later in 1841 so the age at death does fit.
If parents weren't married, the legal surname of any children born would be that of the mother.
Annette