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Daenerys has Varys on her side who is some kind of master spy.

She also has Bran who can scout for her easily.

She was up in the sky from where she should be able to see Euron's fleet where they were hiding before she was in range of the scorpions (if the ships could shoot at her, she should be seeing them as well).

She has a strategist Tyrion who knows about the existence of this dragon-killing weapon when Bronn attacked her during Battle of the Gold Road.

How is it believable that Euron could ambush Daenerys so easily?

The official answer is that Daenerys forgot about Euron and thus was easily ambushed.

Please watch the Inside the Episode for the full justification of this baffling writing.

I think it's well established that the writing on this portion is not as well thought out as it could be. With that said, I think something of a frame challenge is warranted here because I'm not sure we should describe the ambush as being easy and I think some of your assumptions may not be fully supported by the show.

First, it didn't seem easy.

Euron, while a horrible, horrible person, has generally been portrayed as competent. He had a small fleet of ships specifically prepared for the threat they knew they were going to face, waiting in the best ambush point you could ask for, along the route he knew the other side was going to take. There was no indication of how long they were waiting there and it is entirely plausible that they had been waiting for weeks to pull off an ambush.

This doesn't strike me as being something they pulled off easily, but a competent (if evil) military leader using proper strategy to lay out a well-executed ambush at a choke-point. Perhaps Daenerys and her companions should have seen it coming and been ready, but this would have been a reasonable strategy by Euron even if they had known ahead of time. And it might have been harder for Daenerys and her group to know ahead of of time than you make it seem.

Second, Daenerys likely had fewer resources to gain this information than you suggest

I'm not sure that Daenerys had lines of information that were as good as you suggest. You suggest that Bran should have been able to scout, but he doesn't seem able to see the future and doesn't seem inclined or able to take the initiative. He also doesn't seem overly fond of Daenerys and probably would have been in Winterfell with his sister rather than travelling with the boats. In other words, Bran would only have been useful if the ambush was set up before the boats left Winterfell and someone thought to ask him in a way that convinced him to answer.

As for her flying low enough to not see them, that does seem like a tactical mistake on her part, but she is well established as making a lot of tactical mistakes. (The battle of Winterfell was filled with tactical blunders for example). Also, one of her dragons was injured. The injured one might have had a hard time gaining altitude and she didn't seem inclined to separate them. While it remains a mistake not to take the able-bodied dragon both higher and further ahead for scouting purposes, it is one that had a reason behind it.

Tyrion should have warned her about the possibility of an ambush, but a general knowledge that it is a possibility over a long journey doesn't tell you much of use. She should have been more wary, but she was established as overconfident and poor at tactics.

Varys was a skilled spymaster, but that is only useful if you have access to a network of spies and his was disrupted repeatedly.

Somewhat sloppy writing, but justifications are available.

So, yes, I think the real answer is somewhat sloppy writing (at a minimum they should have shown Tyrion providing a general warning of the possibility of an ambush), but in-universe reasons for why they could be taken by surprise do exist.

While I may feel that it was a bit weak that she didn't notice, or at least react to Euron firing at her...

Common sense approach:
Dany: She is distracted by watching her Dragon heal after the battle, probably a bit distracted because she isn't in the North anymore and isn't second fiddle to everyone else in the North.

Everyone else: Last they heard, Euron was still in King's Landing. Vaserys' spy network is likely broken a bit, since the entirety of the North was at Winterfel for the battle, and we know it's a 2 weekish journey from KL to Winterfel.

And there is a long naval history of fleet's hiding on the farside of islands to ambush other fleets. It occurred as recently as WWII in the Pacific, and planes are as close as we will get to dragons (the A10 not withstanding).

Really everyone has their guard down. They are all tired from TBoW, Vaserys and Tyrion are extremely distracted by Dany's growing mental decline and Jon being a Targaryn. The Unsullied aren't a spy network and the small fleet they took to Dragon's Stone didn't include all of the unsullied, only a small number of the remaining. I also don't think they were ever established at being master seafarers, at least nothing close to the Ironbound.

Ultimately, by the time they got close enough to Dragon's Stone to be able to see the otherside of the island from a Dragon, it would have been too late.