Given the time this show was made I bet there was a night when people were sitting around snorting coke and saying "wouldn't it be funny if we made the robot look like a little penis?" and then laughing like Beavis and Butthead.
This could also be a subconcious thing. In an encyclopedia of science fiction and fantasy movies it says about "Star Wars" that R2D2 is penis shaped and is supposed to represent C3PO´s differentiated sexuality which is also why C3PO is the only one who understands what R2 is saying. First I thought this is nonsense but then it made me suspicious when it striked me that C3PO repeteadly calls R2 his counterpart, and I think this sounds somekind fittingly to this theory.
So, maybe it´s the same thing with Twikkie: Perhaps, on a meta-level he is supposed to represent Buck´s penis.
But in addition, I wonder if Dr. Theopolis and Twikkie are supposed to bring the topic of homosexuality into the show in order to handle this subject which was quite present in the 70´s and 80´s. Think about it: Both robots compliment Buck a bunch of times. In the pilot, Dr.Theopolis even praises Buck´s good looks and Twikkie says- as also later in the series: "What a guy!" Of course, Twikkie´s head would fit perfectly into this theorie.
Interesting analysis. Some of Twiki's dialogue would lend itself to supporting your theory. For example, when Twiki makes comments about women ("What a bod!"), it could be taken as something Buck's penis would say if it had a voice.