Extremely poor. So bad that an ad blocker becomes basically necessary. Ads are usually put into a frame. Within it is usually a mishmash of JavaScript, blank spaces, broken buttons, images labeled with hashes, and other useless gobbeldygook. You are correct, screen readers cannot decipher any text from an image unless we use optical character recognition, but let's be honest, nobody is going to go through that trouble just to read some ad coughed up by a google server. Sorry, but those ads are getting blocked. I hope you don't take it too personally. I'm harsh about it because I've had to slog through so many of these things in the past. Most big companies consider accessibility a feature, so it's no surprise that some tiny ad company wouldn't even consider the possibility that someone without vision would come across their content.

As a blind copywriter it's incredibly frustrating that usually the text is embedded in the artwork as described here. I always had to OCR and hope for the best, or sit with a sighted friend who would be agape that I cared about the words in advertisements. Building a swipe file is much harder. Lol