Why do characters in "That 70's Show" mention their parents by their names?

In some cases it does not always mean disrespect, as some parents may encourage or allow this in order to provide trust and create more equal footing by attempting an anti-authoritarian approach to parenting.

However, in the case of That 70's Show it most likley is meant to invoke disrespect or a breakdown of previous American family structure, because the 1970's period is about the decline of a movement of young people (hippies and flower children) rebelling against the establishment, which is furthered by the backdrop of The Vietnam War.

An Anit-establishment view or belief is one which stands in opposition to the conventional social, political, and economic principles of a society. The term was first used in the modern sense in 1958, by the British magazine New Statesman to refer to its political and social agenda. Antiestablishmentarianism (or anti-establishmentarianism) is an expression for such a political philosophy.

In some ways That 70's Show sometimes mocks "Hippie culture", because like in real life, it became a part of mainstream culture, becoming marketable, proving the lack of organization from the 1960's birth (Summer of Love) a failed venture, but still pushed for other sociopolitical movements, such as the sexual revolution.

One way to rebel against the establishment would be to break the social norms between parents and children. However I do not recall if this was true in each case of every character that did so on the TV series and of course there is loads of irony coming from all directions of political correctness of such a debate.

I believe they do it to add a sense of adulting and rebellion to the character's personalities.