Is there an objective level of quality that one must achieve in order to create manga/begin cartooning?
Firstly, in reference to your title, &quot;in order to create&quot; no level of skill or experience is necessary at all - you simply have to start creating. This applies to things ranging from making a simple drawing to making a film. The fact that you know what you want to create shows you have a sufficient idea of what that particular thing is and/or what it means for you (in your case manga).
As to the body of your question, you'd need to define what you consider &quot;doing things seriously&quot;. Assuming you are not merely talking about personal joy and satisfaction (which is a different conversation), I am guessing that with 'seriously' you mean 'professionally', or at least 'worthwhile in the eyes of others' (leaving a similar amount of interpretive freedom for the word 'worthwhile').
Nevertheless, ultimately it is about audience: if you can find an audience for your work, you may consider it serious enough.
And if you want to make a living creating manga, you need an audience that's willing to pay you for your efforts.
Getting an audience is a different matter, but you can start by creating profiles for online platforms like Instagram and Deviant Art, creating a personal website, or looking for (virtual or physical) magazines that publish (amateur) comics. The higher the quality, the easier you will grow a following.
Credentials and education play an important role in showing others that you have made efforts, and have the experience. As you can deduce, not being able to produce credentials is by no means proof of not having put in the practice. Similar to job applications, I feel we as a society have become so accustomed to proof-in-writing, that even though it has its practical benefits, a lack of it is often considered a deficiency in skill and experience, or suspicious at best.
Do your best to create what you want to create. Keep trying, keep learning, and remain critical. The rest will likely follow.