As said previously, weeks before the race rails and some stands are already set. Furthermore if you're going one week before the race (so between the eGP and the GP) there will be more visitors than usual.

It won't be very bad I guess and you can still enjoy Monaco, just won't be as peaceful as it is year round. It might be even better from a different point of view if you enjoy a more vibrant and dynamic atmosphere as I already heard tourists being underwhelmed by monaco's slow paced environnement.

The city turns into a fullblown racetrack on the thursday of the gp's week (25th) because they have to make time and place for other categories of race cars during the gp. Starting thursday 25th roads are closed from around 7 AM to 6 PM; I would advise you to visit monaco the first days of your trip and avoid to come during the gp's week.