Why did Marcy look more lesbian as Married with Children progressed?

The same reason that the most rabid anti-smokers are former smokers.

Jefferson was weak and pathetic and took every opportunity to laugh at Al Bundy's insults directed at Marcy.

They did this to make Marcy a better foil for Jefferson. As r2 pointed out, Jefferson didn't have Marcy's back. That's because he married her for her money. He was a kept man, who only had his looks. He didn't love Marcy, & that would be sad & unfunny had she not evolved into a hard harpy.

Remember when Steve left Marcie, for a season or so, Bud thought Marcie was hot and tried to bed her.

She started looking more butch around just before she came out publicly. I think it was her way of easing the viewers into her new "self". Ellen Degeneres did the same thing back when she came out.