Tiramisu is a famous Italian desert made of, among others, mascarpone and coffee.
As an Italian desert, I believe that an Italian roast of some blend is best to match the rich mascarpone and alcohol tastes.
Is there a "standard" roast or blend combination?
From a cooks perspective, I think it should be "espresso", or "caffè" not "coffee" in any good tiramisu recipe, which in turn would already suggest a type of beans and roast.
Note that the Italian term caffè denotes the strong black liquid you get from an esspesso machine or a moka.
The coffee in a tiramisu has to be concentrated to supply enough flavour without soaking the ladyfingers or other bisquits used. (The same principle applies when you use a liquor.)
When choosing the beans /the roast, the same rule as for wine applies: Use what you drink. And seriously, unless you are making an insane amount of tiramisu, you will be using only very little of one pack of coffee.
I use whatever I choose for my Espresso (if I feel rich, I'll splurge on something from my local roaster, in a pinch the usual suspects like Illy or Lavazza, sometimes even a Starbucks Espresso roast cough) and call it a day.
So in sort, get a classic Italian espresso roast, prepare it authentically and you should be fine. In a pinch, use another coffee, but make sure it is very strong. You'll need a few tablespoons tops for a standard tiramisu recipe.
Consider also using a strong preparation of instant coffee or Turkish coffee as an alternative.
This will allow cooks in an "under-equipped" kitchen (said in jest only!) to prepare a coffee of as strong a preparation as you'd like, in the absence of moka or espresso machine. It will also help with the temperature issue; usually you'll want the prepared coffee to be cooled. You can also use some of the coffee powder as a dusting/garnish in addition to cocoa powder.
As for the roast level, I think this is more personal preference. Your options are limited in commercially available instant coffee powder, but the sky is the limit with other preparations. For tiramisu I tend to prefer a very dark roast (French roast, or Italian roast like @Stephie recommended), which I find to be a good complement to rum, but any roast will do. On the other hand, a bit of bright acidity from a lighter-roasted American coffee would probably be a great complement for the creaminess.
I've also seen a coffee liqueur (e.g., Kahlua or Tia Maria) used in part.
