Not at all. It is a straight analogy.
You said "I do have a question about atheism in general. What is it really? I thought it was not believing in the existence of a god and/or any spiritual being? If that is the case, why the 'F God' to something that doesn't exist? All that really says is atheists do believe in God but hate his guts?"
The direct answer to your question is "No, one does not have to accept the existence of a fictional character to despise him."
But instead of a straight answer, I used an example. Any example of an annoying fictional character would do. Janice from Friends (Oh. My. Gawd). Jar Jar Binks. Allah. Moroni. Zeus.
When an atheist says "F-God," it is not a tacit admission that we believe God exists. Rather, it is an indication that you have imbued this fictional character with traits that are frankly contemptible.
A few years back I had a thread going called "Are you more moral than Yahweh." It looked at God's attributes and "morality" as described in the Bible and compared it to the values we hold in common today. It was not an admission that God exists and a finding that your morality is an actual improvement over his. Rather, it was a call to recognize that you don't consider him to be the arbiter of morality any more than I do.
My way was just more fun.
In short, no, the Steve Urkel comparison is not a strawman. It is a perfectly fine example of the principle that for some fictional characters, the fact that they are fictional is their only redeeming quality.