Answering your question:

Are booleans mutable in python?

Yes and no. Variables that are assigned a boolean value are (probably always, and definitely in this case) mutable, yes. They're also not restricted to being assigned boolean values, as variables are not staticly typed.

But the booleans True and False themselves are not mutable. They are singletons that cannot be modified.

Looking at your actual code:

if success = 1:

Is not valid syntax, you want a == there. Also, idiomatically speaking you should not use 0 and 1 for success and failure values you should use True and False. So you should refactor to something like this:

def update(request):
    success = False

    try:
        product = Mattress.objects.get(id=id)
        success = True
    except Mattress.DoesNotExist:
        pass

    if success:
        return render_to_response("success.html")
    else:
        return render_to_response('failure.html')