The impact of shiny looking design on user uptake

> I had made the design simple just to convey the 'Simple' theme

It doesn't say "simple" to me. It says "not too effort went into it" to me.

Don't get me wrong, I'm NOT judging how much effort actually went into it. I'm actually a user of getsimpleform and happened to recommend it in the formspree post too. I'm just saying that your design doesn't show the "simple" side. It comes off as something that was just cooked without putting in a lot of effort. So, yeah, a design overhaul might work for you.