For recreational and casual skiing, non-certified knee pads can be perfectly fine. These pads skip the formal European safety testing (EN 1621-1), which means their protection claims haven't been independently verified — but that doesn't mean they offer no protection. Many use decent foam padding that works well for typical resort skiing bumps and bruises.
The trade-off is that you're relying on the manufacturer's word about how much impact the pads can handle. If you're mostly cruising groomers, skiing with kids, or just want some cushion for occasional falls, that's usually adequate. Just check that the padding feels substantial and covers the knee well.
Where you'd want to reconsider is if you're skiing aggressively — racing, hitting the park, or charging steep terrain where impacts are harder and more frequent. In those cases, certified pads give you a guaranteed minimum level of protection that non-certified pads simply can't promise.