Cibulkova had a slump, just as Konta is now. No idea what she (Cibulkova) was before.
Plenty of good players go through a slump; it's whether they can come out the other side and return to what they were before, which marks a good player.
As I said though, Konta was unknown on her rise up the rankings. She had a way of playing which worked well for her, but once other players worked her out and her weaknesses she hasn't looked anywhere near the player she was when she got to the US and Wimbledon semis. In short; she's been found out.
If she's a good a player as her higher ranking suggested, then she'll find a way to improve other parts of her game that she can use to change things up; keep other players guessing. If not, then she'll likely just float around 50-100 for the rest of her career; just as Heather Watson does.
Watson shows some really good play at times that suggests she could be much higher rank than she is. Her trouble is confidence and she loses it too often. If she is confident, she can be a very good player. If her confidence is low, she gets beat easily by similar and lower ranked players.
Konta on the other hand seems full of confidence, but she is possibly a bit arrogant. I've seen that has been suggested. Maybe that's why she has trouble adapting her game; she doesn't like criticism and can't take that onboard and use that to look at her game. She's going to have to change that if she is going to get back to top 20 ranking.
That's my view anyway.