Is 1.3k MAU a good indicator?
First of all, I am a developer and my thing is building products and I have a large knowledge gap when it comes to almost everything else about growing products . I built an app a couple of years ago which gets signups through app stores search engines, GA shows 1.3k monthly active users, is this a good indicator that the app can be somehow monetisable as it is?
Try also to consider the following:
In theory, you would have a profitable app if the lifetime value (LTV) of the users is higher than their cost of acquisition (CAC). In practice, you will want to make that money back within a few weeks/months (CAC payback period).
You are already acquiring users organically. That's a good sign!
Really depends on approach. But you can search for in-app ad revenue calculators to get an estimate.
https://www.yaftamobile.com/tools/mobile-ad-revenue-calculator
This one suggests you'd be earning less than a $1 per day.