ok to make it short; my senpai called me tennen; I am forgetful person and I always do a lot of mistakes so I assume he means I am stupid based on its dictionary meaning but according to my other coworker it is not that bad compared to being called あほ or バカ so I am still confused.
I think it describes a person or person's character that the result of acting without intended attempt is out of common sense of ordinary people; especially when the result is biased to the heartwarming or acceptable side.
天然 can mean "natural", but in this case it's "airhead" as a shortened version of 天然ボケ "natural fool". It just means that you're not actually dumb or something, but maybe exhibiting a careless nature? Something akin to a lot of "oh yeah..." moments.
I remember the meaning of 天然 described to me as a person who makes silly mistakes by my teacher. An example would be a person who looks themselves out of their car or apartment regularly.
I think 天然 roughly means "naive".
Calling someone 天然 is not offensive; 天然 is not offensive word