To think that lunch in a restaurant should not be eaten in a bikini

[smartyparts]

regardless of whether you're fat or thin?

Is it bad manners? I think so.

With apologies for link to the Sun, but at least it's not the DM!

IMO it's bad form not to cover up when eating/drinking in a restaurant/cafe/beach bar.

It takes less than a minute to slip on a sarong or pull on a pair of shorts and t-shirt, thus preventing unsightly marks from plastic chairs and guarding against hot metal chairs and possible food/drink spillage, to say nothing of putting other diners off their meals.

A picnic on a beach may be an exception, but even then I would cover up with a towel or wrap as handing food around or eating legs akimbo is not a good look.