In a 1994 episode of the Simpsons they make a lot of jokes about the Republicans party being evil. When did it become culturally normal to characterise the party this way?

This question runs up against the 20 year rule, not because this started happening 20 years ago but because it's still on-going. The characterization is multi-faceted, but I'd argue that it it stems from the partisan divide over Unions.

Unions have historically seen corporations as antagonists (see the Battle of Blair Mountain and other union-busting activities in which companies used private and/or state-sponsored violence against Union organizers) and they've also historically been deeply connected with Democratic politics especially since the New Deal.

Taken together, this sets up the two major parties in the United States as ideologically divided over the question of "can we trust corporations to do the right thing," first with regard to worker protections and then, eventually, everything else. Democrats, dependent on Unions for political organization, will say no. Republicans, dependent on corporations for the same, will say "yes."

So the question then becomes less "when did it become normal to characterize REPUBLICANS as evil" and more about "when did it become normal to characterize CORPORATIONS as evil."

For that we can turn to some more contemporaneous sources. This is what I mean about the 20 year rule. Since this question is fundamentally historical, any primary source answering it is likely to observe the 20 year rule itself, so to keep within the rule ourselves, we'd be looking for a 20-year-old source discussing pop-culture from 20 years before it was published. I'm sure they're out there but I'd argue it's not necessary in this case.

This article from the Atlantic only briefly touches on the "beginning" of the trend of "corporations are evil", but it does document a shift from government-centric-dystopian-fiction towards corporation-centric-dystopian-fiction. Not surprisingly, this begins to take hold as the memory of the government's near total control of daily life fades following the close of World War Two. Post war America is one in which private companies rise in importance relative to government and, with that rise comes a change in the tone of fiction which worries about what life might be like under a corporate boot-heal.

By the 1980s this is so well established that films like Robocop aren't even pretending to avoid comparisons to their contemporaneous political environment. Rapacious, amoral, profit driven corporations are shown pillaging public goods (like law enforcement) regardless of the human suffering caused. These fictional representations of Republican politics land, in large part, because the GOP is, by this point, committed to Reagan's message of "government is the problem" and expressly endorses a political position in which there are very few regulations on what business can do.

This plays out in ways which cast Republicans as almost cartoon-like villains in several instances, most notably with respect to the tobacco industry. Today the tobacco lobby gives almost exclusively to Republicans but in 1990 (which is as far back as I can pull data from OpenSecrets) both sides took money... though the GOP did take slightly more. It wasn't until the late 1990s that the GOP felt that the political risks of taking money from Tobacco companies outweighed the benefits by which point about 9 in 10 people understood tobacco to be harmful.

By this point we're a little ahead of the 1994 target date but the broad outlines are still important: the GOP's anti-regulation position put it in a position of fighting for causes that many Americans saw as overtly harmful especially from about 1980 onward.