There is no such number - on Amazon, it's always relative to your competitors on that keyword(s). If you're in a niche where your conversion rate is better than the top competitors, then that would be considered a good conversion rate.
It means nothing to have a 25% conversion rate if your main competitors have a 30% conversion rate. Your competitors will win the top organic ranks, visibility, and ultimately sales.
The opposite is true where a 5% conversion rate would be considered "good" if you're competitor's conversion rates are 4%.
How is this relevant to PPC? When you're analyzing your PPC by conversion rate, be careful what you consider to be low or high before making any meaningful bid or targeting adjustments.
I have seen sellers negate targets because their conversion rate was 5%, but what if that 5% was a good conversion rate for that keyword?