We periodically get Spam (i.e. unsolicited) emails about one or other of our sites, telling us that there is something wrong, outdated or needing improved (but never specifying what). They all go straight into junk.
If you're going to try this approach, I would suggest you do it very carefully. Make the e-mail as personal as possible, mentioning the site by name, and addressing the owner by name, if that information is available. Anything to make it clear that this is a personal contact and not a Spam e-mail you have sent to hundreds of recipients.
Then be very careful about your wording. Nothing is more likely to put somebody's back up than having a random stranger criticising them or their business. If the site is built on Wix, there's a high probability that the owners did it themselves, so telling them it's rubbish is not calculated to get them on your side.
If there is an instance you can point to and say "I notice [this part] of your site is out of date. If you are looking to update the site, then ... ", that might be a tactful way in. Keep the initial contact general, and wait to see if they are interested in learning what you think could be improved before you go into detail.
It's not an approach I would use, but if you want to go down that path, I'd say go slowly, tread carefully and be tactful.