Why isn't he called Dr Freeze? He is a doctor. Normally a doctor wouldn't go out of his way to be referred to by Mister.

Maybe his doctorate is in a field unrelated to turning things into ice?

And not every villain named doctor has real credentials. Dr. Doom never finished his degree when he had his little accident and he ran into the mountains. Doesn't stop him from referring to himself as a doctor though.

The major reason lies in the way past. In the beginning he was known as Mr Zero (as he was not a doctor just a scientist). He stayed that way till the Adam West Tv show came along and said the name was too cheesy even for them so since he had cld based powers they decided that Mr Freeze sounded best for TV and this from that day on his name was changed to reflect this

Still better than Mr. Zero (his original moniker).

Batman calls him by his first name anyway.

Dr. Freeze sounds like a Popsicle company.

Pretty much as annoying as if they were to change Brother Voodoo to Dr. Voodoo. Oh wait, some idiot did that.