well, in the sense of "a guy sitting in a dive bar drunk in 1978 slurring about how he was cheated out of his fortune in 1930", "loser" doesn't seem inappropriate. Also, "selling" his daughter for $100 after driving nearly falling-down drunk, swerving and bashing into things, doesn't help his cause..
I just checked the pilot script - David Jacobs' first description of Digger is "not a derelict, but clearly not a solid citizen, either". Later, Cliff (in a line that didn't make it to the screen calls him "a drunk pretending to be drunker than he is... for drinks".
I guess it depends on the definition of a loser. Sure he didn't abandon his family like Rebecca, but without his sister propping him up he likely would have been long dead before the show started. Would he really have been there to kiss little Pammy goodnight (when not out on a multi-day 'bender', that is) without Maggie? Someone could have a good heart and still be a loser, right?
(I didn't realize what a passionate Maggie advocate I am!)
The Keenan Wynn version seemed a bit more together than David Wayne's portrayal.. But I liked how David Wayne spoke with the cadence of a man who felt like he should be important: "Cease! I have business to discuss."