A result, not a description
A clean sheet describes an outcome, not the process that led to it, and the two can be quite disconnected. A defence can be repeatedly opened up, concede several clear scoring chances, and still keep a clean sheet purely because the opposition finished poorly or a goalkeeper made exceptional saves. Judged purely on the scoreline, that would count as a defensive success, but judged on the underlying pattern of play it was closer to a defensive performance that got away with a great deal, and the two readings lead to very different conclusions about whether the same approach will hold up again.
What to look at instead
A more informative question than whether a team kept a clean sheet is what quality of chance it allowed the opposition to create. A defence that concedes few shots but those shots come from close range and unmarked positions is in a more precarious position than one that concedes more shots but almost all of them from difficult angles or distances. This distinction, between the quantity and the quality of chances conceded, tends to be a far more stable predictor of a defence's underlying performance than the clean sheet count alone, which is heavily influenced by finishing on the day.
The best defending is often invisible
The most effective defending frequently prevents a dangerous situation from occurring in the first place, through positioning and anticipation that stops an attack before it reaches a shooting position, and this kind of defending produces no dramatic clearance or block for a highlight reel to capture. It simply results in the ball being recycled back to the opposition with nothing having happened, which is precisely why the players and units regarded as the best defenders by coaches are not always the ones most visible in statistics built around blocks, tackles and clean sheets.