Is card counting/watching a key skill for an expert Catan player?
Yes, card counting is a key skill for expert Catan players. First, most expert Catan players will be counting your cards and your opponents' as they pick them, with the only random variable being any possible development cards that you draw from the deck, or cards which you steal from another player when you role a seven. But as they will be counting your cards, this would immediately put you at a disadvantage if you are not counting theirs.
Once you can reconstruct an opponent's hand, you will know who you can trade with, what people need, and whether people are a threat to steal your spots for settlements and roads.
Although there are probably a very few world class players who are not that great at tracking cards, the vast majority of those who play competitively or in sanctioned tournaments are. And if they are not great in tracking cards, then they would have to be exceptional in another aspect of the game, like strategy, or be extremely lucky.