25% each mushroom compost and manure is a bit excessive in soluble salts for seed starting and small transplants.

Nutrient overload of niitrates? Good mix for celery maybe

"Planted zucchini a few weeks ago."

Were they direct seeded, or are those transplants? I note that the newest leaves appear to be greener, so the seedlings might already be recovering. It is possible that all they need is a little time, and warmer temperatures. If nothing changes after a week or two, there might be other causes... keep us posted.

If the manure is properly composted, you won't be having a problem with soluble salts from that. But yes, mushroom compost can do that. Did you get your compost and manure from trustworthy sources? Herbicide-laced manure and compost is getting more common these days.

You don't say where you are. Are the temperatures comfortable, or is cold stressing the plants? Too much sun? If transplants, were they hardened off?

True. Could be rough transplant stress. Not recommended to transplant cucerbits although I have no choice where I am. If my transplants are too big they often look similar although not quite that bad and often rebound quickly

Ideally I like to transplant cucumber bits when the first true leaves appear and are no larger than an inch each

So likely mineral overload of some kind it sounds like

Have they ever been sprayed with anything when it's been warmer than 80

I hate that mixture of ingredients you're using!