Eh, I tried exactly this for a "thought bubble" project I had. Figured a landing page and $200 on ads would be the best way to gauge demand before writing any code.
Seemed like a great success - 50 or so e-mail signups, time to roll up my sleeves and start building! Until I looked closer and realized that every single signup was from a clearly fraudulent address. There's absolutely zero chance that they all just happened to be female, nicely formatted [firstname].[lastname]@gmail.com addresses.
Using ads to dip your toe in the water might have worked ten years ago, but there's just way too much fraud nowadays. I think you're better off finding your first ten users manually and asking very nicely for their time and feedback.
That was a few years ago, but I launched my app on the Play Store recently and noticed the exact same pattern. Definitely fake, too, because accounts were created in Firebase without user profiles (i.e. the app was never launched). I assume there are bot farms scanning new releases for certain packages (e.g. Firebase) and creating fake accounts.