Drumpf, as HBO host John Oliver discovered, was the Trump family name before they moved to America and changed it. Since that segment aired, many people have referred to the candidate as "Drumpf."
Personally, I think this is somewhat silly and really immature of the Kasich campaign. There needs to be a modicum of respect among the candidates, and that includes calling them by their proper names. Also, I don't understand how "Drumpf" is an insult--it's not like Donald himself changed his name. Many families, mine included, changed their names upon coming to America. Trump just got lucky that "Trump" is a heck of a lot more regal sounding than "Drumpf." Additionally, Kasich was the one who called for the candidates to act like adults--he could start with himself.
It'd make more sense for the Kasich campaign to care more about the delegate race than the last names of his opponents.