About 20% of NFL players in any given season were UDFA's at one time. 11 of NFLN's top 100 players of 2011, as selected by the players, were UDFA's (Harrison, Cribbs, Welker, Gates etc).
Regardless, I do think Madden has missed the mark here. They have the right idea, but it's been implemented in a clunky way that doesn't really emulate the real NFL. First, the udfa's they have in the game are probably much worse (based on ratings) than the ones that actually make NFL rosters every season. Second, they probably have too many of them making teams. While 20% of the league is made up of udfa's, that is a cumulative effect where 50-70 udfa's make teams over a number of years, not 100+ in one season.