ラッピングバスって英語でなんて言うの？

Bus companies in cities or towns frequently sell advertising space on their vehicles as a means of increasing revenue. The vehicles may have a number of ads from different companies, or just one ad promoting just one company.
Eg. "Look at that bus with a Pepsi paint job!"

There are several terms you can use to refer to a bus wrapped in advertising.
Bus advertising
Campaign Bus
Mobile advertising

Mobile advertising can also refer to a banner being pulled by a vehicle or airplane.
Campaign bus, usually refers to a political or social campaign project.

This is basically a system of advertising to reach an audience who are stationery or on the move. If the advertising covers a large part of the bus, i.e. both sides and the rear end, you can call it a 'The Full Wrap' Bus. Some buses only have 'The bus back' (advertising on the rear end of a bus), 'The Kerbside' (advertising to hone in on pedestrians), 'The Roadside', etc.

A campaign bus (battle bus in the UK) is a bus used as both a vehicle and a center of operations in a political campaign, whether for a specific candidate, a political party, or a political cause. A campaign bus can also transport members of the press covering a candidate's campaign.
Mobile Billboard means an on- or off-premise advertising sign attached to a vehicle or trailer that is used for the primary purpose of advertising and that moves with pedestrian or vehicular traffic or is parked at specific locations. A vehicle which advertises the company of its primary use is not considered a mobile billboard.

▪ Bus wrap signage
This is promotional signage that is wrapped on a bus for advertising.

▪ Bus advertising
This is promotional advertising that is wrapped on a bus. Also called bus branding.

Honestly it depends on what kind of bus it is. For example: a "Greyhound Bus." is a bus used by the Greyhound company, and it typically goes from city to city. A "City bus" is the local bus transportation used within city limits.

Bus advertising is the adverts that are printed or on a bus that people can see, this can also be called 'mobile advertising' as the bus moves and does not stay on one place although this can also be a board or advert that is pulled by a vehicle on the move