In (mostly UK) drug parlance, a person who rolls a joint with most of the cannabis at the top, knowing that they will get a monster hit when they light it up. Toploaders are never popular.
Sasha Gabba Hey! says that some people top load a joint when the end is intended for someone who doesn't like tobacco; I suppose, yes, then that could make you more popular as a toploading fiend.
Some early video recorders are known as top loaders, because they, um, load tapes in through the top; top loading tape carriages are simpler and more reliable than front loading types. Front loading VHS decks are popular for reasons of convenience rather than any technical fact. The Ferguson VideoStar came in both top loading and front loading varieties.
Toploader are also a band.