Well usually when I see a home on the gallery that doesn't look particularly good, I remember that someone who built it thought it looked good so I just shrug it. So really I don't think it is possible to make a horrible lot.
Sorry for another comment but after looking at other Italianate buildings I also think a good idea would be to just make the tower a flat roof with a frieze on the top. Again sorry for another comment!
I know you were looking for a Mansard roof but maybe try a traditional Queen Anne pointed style roof on top? I feel as if it wouldn't fray too far from the Italianate style and it might be the best option for right now. Hope this helped! Edit- I meant wouldn't fray too far not would.
I have noticed that with all the Sims games they tend to focus on a specific set of themes when it comes to architecture. And in The Sims 4 I can honestly say that some VERY good themes that work well are Victorian, Mid-Century, Modern, and Italian Villa. With alittle work, I am sure some others could be pulled off, but…