Hi Ryota,
To answer your question yes, these phrases are informal but correct. People often text back with things like "is okay" or "Am good." They're casual ways of saying "it's okay" or "I'm good." In everyday conversation or texting, it's perfectly fine to use these phrases because they're straightforward and convey the message quickly. For instance, if someone asks how you're doing, you might reply with "Am good, cheers!"
I hope this answers your question :)