success_message in DeleteView not shown

I think this issue in the Django issue tracker should answer your question.

SuccessMessageMixin hooks to form_valid which is not present on DeleteView to push its message to the user.

It also gives an alternative way which works for me:

from django.views.generic.edit import DeleteView
from django.core.urlresolvers import reverse_lazy
from django.contrib import messages
from .models import Thing

class ThingDelete(DeleteView):
    model = Thing
    success_url = reverse_lazy('list')
    success_message = "Thing was deleted successfully."

    def delete(self, request, *args, **kwargs):
        messages.success(self.request, self.success_message)
        return super(ThingDelete, self).delete(request, *args, **kwargs)

SuccessMessageMixin was not used in the delete-view (but I do use it for the Create and Update views).
Hopefully this will be improved in later versions of Django (see issue for more info).