yummines hit the nail on the head, but I'd like to add – it is a highly-fetishised stock character which has lost its basis in reality, now intended solely to cater to Japanese nerd audiences and similar fandoms.
There are no real-life tsunderes, there are just those people we call "moody". For contrast, see how negatively Western fandoms react to moody characters in their products. I can recall several instances from Dragon Age (Morrigan, Velana, Fenris) who would be classed as "tsundere" were they from Japanese-origin series. As they are not, they are merely considered moody jackasses even in spite of the personal stuff they are respectively dealing with.
The entertaining aspect comes from parody versions such as Noire from Hyperdimension Neptunia. The other characters frequently point out how ridiculous her actions and behaviour are and this is primarily intended for lulz, but it also has the side effect of showing how stupid and awful tsunderes would be as actual people, in that they are incredibly difficult to like.
"Tsundere" is also a loanword and I strongly suggest you alll avoid using it where possible, since we have several terms and expressions in our respective first languages that already describe what it is.