That's what really sickens me about most people who take part in the competitive scene. They only use characters because of their tier position and not because they genuinely like them. Only a few actually happen to like Diddy Kong as a character.
And what do they mean by "Nintendo's Lightning"?
"X character is Y company's/franchise's/creator's Lightning" is a meme originating from the Final Fantasy franchise, specifically the XIII saga.
The character in the spoiler tag is Lightning, one of the main protagonists for the XIII saga, and the default poster child and representative of the series. The XIII saga only spans 3 games (and some novels if you want to be REALLY technical), and Lightning has appeared in other games besides these as a cameo (FFXIV), or as a representative of the XIII saga (The Dissidia series is the Final Fantasy equivalent to Smash Bros, being a fighting-action game that pulls characters from the different FF universes together - Lightning is the playable representative of the FFXIII universe).
In XIII, she was one of the main protagonists, along with 5 other characters.
In XIII-2, she was a supporting character, and showed up for no more than an hour of the story's 24 hour run-time (she also narrates the story, but it's pretty infrequent).
The third game "Lightning Returns: FFXIII", features Lightning as the main character again, and the only playable character.
However, Lightning has a stigma with some people in the Final Fantasy fandom, in that people who don't like her or the FFXIII games feel she is forced down our throats because her creator, who has a high position at Square Enix, loves her so much.
Thus, when fandoms feel a character is forced down their throats because a higher up created them and loves them (in Rosalina's case, this would Yoshiaki Koizumi, who actually is in a high position - he's a producer of the Mario franchise right now, if I'm not mistaken), and they consider that character a "Mary Sue" for some reason, that character is dubbed as a "Lightning".
However, the whole meme is stupid. Not only is it inapplicable to Rosalina (she's undeniably popular all over the world, and she isn't a Mary Sue by any stretch of the imagination), but it's inapplicable to LIGHTNING HERSELF, as Lightning IS genuinely popular, ESPECIALLY in Japan, where she's considered one of the biggest female role models in Japanese pop culture. And she's DEFINITELY not a Mary Sue, as she has a lot of character flaws that make her more interesting (and potentially hard to like at all for some people).
Lol, can you stack that effect?
Unfortunately, you cannot. :\