I personally think anything that encourages/any space that enables women to talk about their experiences and how they feel about them, especially things we're 'banned' from talking about, is valuable and important.
And no, I'm not inexperienced, I've lost count 
but I've never had a male lover who fiddled with his own tits at all times through the proceedings (or at all?) I do it to them and see whether they like it but haven't seen any do it to themselves during sex with someone else (except maybe the Samantha incident which was a bit of role play.)
But IMO it is very harst to tell him you dont like him doing it. You'll make him feel bad and potentially stop doing it, and so he won't get to do this thing he really enjoys.
@CrazyToast It is turning OP off, so damaging her enjoyment. She wouldn't be saying he shouldn't do it ever with anyone, but that she personally wasn't into him doing it all the time, or maybe much at all. Which she has a perfect right to say. If he wants to 'play Samantha' with someone else then he's free to do so.
I got off with someone once who wanked himself and went 'Oh LOOK at how big it is! LOOK at how big it is!' Or one guy who (unnecessarily) would insist on trying to add spit as saliva even when I'd told him he didn't need to. Both of these behaviours I didn't like.
It isn't 'shaming' to say I personally didn't like them (though I expect most women wouldn't be a fan of the 'LOOK at how big it is!' guy at least.
My ex had a micropenis and yes it's maybe kind of shaming to say that didn't do it for me. But to say we didn't like someone's genitals in some way is more of a personal preference maybe.
Women shouldn't have to endure bad sex with people we find unattractive in any way just to be 'nice' or for any other reason. We've suffered enough over the millennia. And talking about these things is important/empowering, allows us to acknowledge and act on how we feel.