Konnichiwa.
Otherwise everything else in your sentence is correct.
And just to clarify, you are saying, "Good day, is this a Transformer?"

Now having said that, it's unnatural to start off with good day. You'd be better off saying:
「すみません、これはトランスフォーマーですか。」
sumimasen, kore wa Toransufōmā des ka?