Mueller had made clear that he did not want to testify and that all he had to say was already in his report. The Democrats insisted. But today's hearing went poorly as even their partisan followers admit:

Laurence Tribe @tribelaw - 18:30 UTC Jul 24, 2019
Much as I hate to say it, this morning's hearing was a disaster. Far from breathing life into his damning report, the tired Robert Mueller sucked the life out of it. The effort to save democracy and the rule of law from this lawless president has been set back, not advanced.

During the hearing multiple Democrats tried to get Mueller to support an impeachment of Trump. But Mueller never gave them that gift. The Democrats should thank him for that. An impeachment process against Trump is not popular:

A July Post-ABC poll found that 37 percent of Americans support Congress beginning impeachment proceedings, while 59 percent do not, with a 61 percent majority of Democrats backing proceedings.