A quick Google search in Hebrew brings up quite a few people named "מנדעל".
Even if Mendl is more accurate to the original,
Mendel deserves to have its own entry, as both people in non-Yiddish speaking countries and in
Israel are using it as a name.
You are more than welcome to (and I think you should) add Mendl to the database as the original form of
Mendel.