Is cognitive impairment as a side effect of kolonopin very common or rare?

Common. Klonopin is a benzodiazepine, commonly referred to as "tranquilizers". This group of meds includes valium, xanax (alprazolam) and ativan. Sedation is a common side effect, and yes if you are sedated you can display cognitive impairment, particularly if mixing these meds with alcohol or other sedative medications.
Common. I don't have a percentage, but it is more common for Klonopin (clonazepam) than other benzodiazepines. It is considered one of the more common side effects, especially in short term use (hence one should not operate heavy machinery on it).