I've made a hard diff map and now I want to make a normal. Is it OK if I take the hard and simplify it instead of making the normal from scratch?

That usually doesn't work well because Normal and Hard diffs have different slider velocities, spacing, patterns etc. so you'd have to adjust all those things anyway, it's easier to just map it from scratch.

Hard and Normal follows two different set of rules, what is essentially prohibited to use in Normal may be acceptable in Hard and what is fine on Hard might not work on Normal, so it's rather more nuanced than a simple "delete some objects".

I'd rather go with doing things from scratch since the two diffs are very easy and fast to set up even with an average amount of skill.