In addition to the blame that Trump fully deserves, the Republican "leadership" of the House (and to a lesser extent the Senate) get a portion as well.
At no point in the campaign did they present any message about what they would do if they took control of Congress. They thought that they could allow the election be a referendum on Biden/the Democrats.
It was that failure to present any message that allowed Trump's shadow to be felt so strongly over the election. Since Republicans weren't talking about anything, the race became about what Democrats wanted it to be about.