But I never hear about Mondale and Dukakis in the media. They are quiet. We hear about Gore because of the environment issues and we hear about Kerry because he was secretary of state.
They've certainly kept a very low profile. However, there was a report on 60 Minutes a few years ago about electroconvulsive threapy for patients battling depression. In that report, the Dukakises were interviewed, because Kitty Dukakis (Michael Dukakis' wife), is a patient who receives electroconvulsive therapy as part of her treatment. This is probably the only time that Dukakis has made a notable media appearance in recent years.
And Gore and Kerry were both featured in media reports last year, in the context of the presidential election. Gore appeared in a virtual chat with Biden to offer his endorsement, while Kerry was one of Biden's staunchest supporters in the primaries and actually went to Iowa to campaign on his behalf. He later was one of the speakers at the Democratic National Convention.
FiveThirtyEight and ESPN produced a short documentary about the infamous tank photo-op in 2016, including an interview with Mike Dukakis himself (although I understand that 538 is not exactly a notable media appearance for people who aren't political junkies like us).
I don't know about Mondale, although it should be noted that he had his own late comeback to relevance when he became the Democratic replacement candidate for Senator in Minnesota in 2002 after Paul Wellstone died in a plane crash.
Dukakis occasionally pops up on the news in Boston every now & then for spending his free time cleaning-up litter on the streets of Boston, but the last time that I can recall him actually being in the news for anything political was when he was the heavily-favored appointee to fill the Senate vacancies created by Ted Kennedy's death back in 2009 & then Kerry's ascendancy to the State Dept. back in 2013, though I guess Deval Patrick must have something against him or something, because he didn't appoint him on either occasion.
As for Mondale, he's definitely kept himself on the down-low. The last things I remember him being in the news for are endorsing Klobuchar because Minnesota, his appearances on Carter Center panels (which, y'know, have generally been newsworthy only because they're team-ups between the earliest-serving President & their VP who are both still alive), & the lunch he had with Biden & Pence a few years back when he accompanied Tina Smith to her swearing-in - conducted by Pence, of course - on the same day that Biden accompanied Doug Jones to his (i.e. "a historic get-together on the Senate floor today as 3 VP's gathered..."), all of which feel like you kinda have to be a political junkie to actually notice & remember having heard in the news.