It seems to have a very negative impact considering how browsers are actively trying to block them along with antiviruses tagging them as harmful. No one ever really mines using their own browsers anyway. It's not a very good look for Bitcoin either because the less tech-savvy seem to think they're planted by Bitcoin hackers and that they all mine Bitcoin.
They're potentially an excellent alternative to ads if they were transparent. I mean, at least the user knows exactly what it's doing, unlike ads which may contain harmful code. On the other hand, even if they were transparent, there's still the issue of the visitor's computer bogging down whenever they use your website, so it may actually turn away visitors. There's also a stigma of cryptocurrencies being associated with hackers and viruses, so people may not want to run anything that's miner related.
Either way I run an ad blocker (uBlock Origin) along with a script blocker (NoScript) so I don't have to deal with either lmao.