It's silly in English too. Perhaps some British readers might be familiar with the name and its history/origins, but for most English readers, I suspect, it just sounds a bit silly, like he has a very tall butt.

Washington, D.C. has the famous Foggy Bottom.

Edit: America has a lot of bottoms, apparently: https://en.wikipedia.org/wiki/List_of_geographical_bottoms

Nowhere as grand as Aunt Mary's Bottom but South Dakota does have a Big Bottom.

"Talk about mud flaps My gal's got 'em."

It's silly, but it does feel like a real name. There are many such names in Britain.

As someone with an English "bottom" as in bottom-lands surname, I appreciate the deliberate silliness of "Longbottom" while leaning into a very traditional British sounding name.

Does it refer to a large valley? (lots of fertile ground) Perhaps, it is a punny reference to him being a pure blood.

Longbottom is the serious version, before that it was Stretcharse.

In French he's called "Londubat", which phonetically means "long FROM the bottom", which is arguably even worse than the English name.