Is it true that a word 航空母舰 (lit. "an aircraft carrier") can be used as an euphemism or a joke for a "woman with big breasts" or "plump woman"? I've seen such a weird translation of a word 航空母舰 in an online dictionary but could not find any actual usage of the word in the described context. Was it a mistake or a kind of a joke?
No, there shouldn't be such relation. I can guess where you might see this weird relation between aircraft carriers and women with large breasts. This connection often appears in warship personification animations/drawings/games, among which the Japanese game KanColle (舰队Collection) is probably the most famous one around the world. 

The reason behind is that authors of animes/drawings often relate the tonnage of a warship (not only aircraft carriers, but also battleships and sometimes even heavy cruisers) to their breast sizes. In a warship personification game, if an aircraft carrier character is designed/drawn with flat breasts, she will probably become a popular topic among players for a long time. For example, among games developed in China, the Saratoga aircraft carrier in the game Azur Lane (碧蓝航线) and Hood battlecruiser in the game Warship Girls (战舰少女) are those who break the breast-tonnage rule.

As a summary, please do NOT try to use this word to describe anyone in real life. Among warship games players, they might understand what you are referring to. But for most people outside this group, they probably get confused, or even worse, treat this as impolite or offensive. We can make fun of game characters in some cases, but never of a true person.