Firstly, both expressions do not mean the same thing.
And really, our brains don’t work that way. Our subconscious mind does not recognize negative prefixes. Such that when someone gets a response of “not bad”, internally, it is interpreted as “bad”, and this starts to influence the feelings of the recipient negatively. Science supports this.
If there was a Likert scale to measure this form of responses, on a zero to five, am guessing, statements like ‘not bad’ will be below mid point. Somewhere between 0 and 2.
My point is, if you want to offer feedback, it cost nothing to take a bold stand, to communicate inspiration and energy. It’s always better than playing on words. You either give a thumps up or a thumps down. Simple. Whatever you do, use the right vocabulary always.