In social proof quotes is it ok to replace "it" with your product name?

I think it depends on the context the person gave the feedback. Was is formal feedback or not? If so then make sure it sounds natural. Don't let your testimonial section sound forced, otherwise you don't just invalidate your testimonials, you turn them into a detractor.

If you put quotes around it, write exactly what they wrote or follow jarrad's advice to make it clear you replaced pronouns.

I've generally seen this done in media by changing "I love it!" to "I love [Moon]!" It means that the speaker was referring to Moon, even though they didn't explicitly name it.

Going to your landing page and looking at the first comment by Emil Moeller, I think it would be appropriate to use:

"[Moon]’s fast and smooth! I love using your app. Was just using it now for the last 20 minutes, and it’s so fast, and gives a great twitter stream like feeling. I’m grateful for the time and effort you’ve put into it. It’s a very enjoyable way to read Indie Hackers."

The link to your app in your post is broken by the way.