In the roblox terms of use, one of the prohibited items is “The use or sale of drugs, alcohol, or tobacco by minors…”
My question is, if a logo has a picture of an adult smoking a cigar (a mafia boss for example), would this be against terms of service in any way?
Yes it would be since it’s promoting the use of tobacco.
The community guidelines as well as developer guidelines say nothing about tobacco or cigarettes/cigars other than depicting its use by underage minors.
The guidelines don’t say a lot of things, but through logical assumption you can likely guess that they wouldn’t want this on their platform. I recommend not stepping into grey areas if you don’t want your game unexpectedly moderated.
If you have to ask if you can put something, you probably shouldn’t.
To add on to some of the previously posted advice, it might be worth to ask yourself what you want to achieve with this specific logo. Are you trying to set an authentic feel for a brand in your game? You might be able to find alternative objects or designs that can achieve that feeling without entering the realm of potential TOS grey area.
Additionally, your example of “(a mafia boss for example)” would break the Terms of Service since gang-related content is not allowed.
Doesn’t jailbreak have a gamepass for being a mafia boss? In the community rules, it states that the developers can’t allow illegal behaviors to happen in the game. There are tons of games where players virtually commit what would be in real life, illegal acts. This makes me assume that as long as your not promoting real gangs or real gang violence, it might be ok?
A mafia boss is sorta a long stretch to a gang. It would be like comparing weapons in games to real life shootings. Yes, they are both related, but it is not the intent and almost anything could be connected to something inappropriate for Roblox.
@Mah_Bucket made a game which had a glass of alcohol in the background of it’s thumbnail, and it was rejected. I believe a cigar in your game’s logo would be a massive no-no.
I think instead of using a cigar, you should use a tommy-gun. Mafia-related guns would be a tommy-gun and it would bring the vibe of Mafia related. Good luck! 
Or have him holding a wad of cash, that’d be fine and emit a similar feeling I think