I forgot to report my AP scores on common app, is it a big deal?

I would email them! But only if you scores are competitive for the school

I would reach out to the admissions offices and let them know what your AP test scores are. Don't assume they will figure it all out unless you have already paid Collegeboard to send your AP scores directly to the colleges. Most people self-report and then depending on the college, they either pay Collegeboard immediately to send the AP scores or wait until they are admitted. Each college has its own policy as to when it needs the actual Collegeboard Official AP scores, to corroborate your self-reported test scores.

Depends on what your scores are. If they're <4, they're most likely not going to make much of an impact on your applications. If they are 4 or 5, you can still alter your application after you've submitted it and it would be beneficial to do so.

To be honest it's not a big deal. Most colleges don't consider your AP scores with too much weight anyway. Unless you had a lot of 5's, specifically relating to your field of interest, but even then it's not a big deal. Your AP score itself is more of a reflection of how well your teacher taught you, as opposed to your actual grade in the class.