No. In Tâi-gí, "哇係" is pronounced "oa-hē", while "我是" is pronounced “góa-sī". Totally different.

Yes, i meant, is it an attemp to use the mandarin phonetic value of the characters rather than the meaning...

to show you're expressing yourself in taiwanese?

例 : someone post a picture of someone else.

that person asks : 系哇嗎?

the other answer : 對, 是你啊!

Yeah, lay people do this. I personally do not think this a good practice.

By the way, "系哇嗎" is definitely not Tâi-gí. The correct expression whould be “敢是我?" See, we do not need the phonetic approximation because

the grammar shows it already.

I don't speak 國語 well anyway you know, but i was curious about this. an internet search with 哇係/系 gives many results by the way.

What is sure is that, i will understand nothing when 台語 is written phonetically like you do