Dumoulin played against every single top player the Penguins faced.
Maatta did not.
Just for reference, here are the players top three forwards from each team each played against most this past season (sticking with just our division since I'm lazy and can't be bothered to do it for every team):
Dumoulin
Washington - Oshie (17:34), Ovechkin (15:07), Kuznetsov (13:34)
Columbus - Dubinsky (26:07), Foligno (23:54), Atkinson (23:34)
NY Rangers - Zuccarello (34:55), Stepan (32:52), Kreider (24:34)
NY Islanders - Bailey (28:22), Tavares (27:42), Lee (22:27)
New Jersey - Zajac (25:46), Palmeiri (25:04), Hall (24:40)
Carolina - Aho (16:16), Lindholm (14:06), Staal (12:37)
Philadelphia - Giroux (28:43), Voracek (25:13), Simmonds (21:00)
Link
Maatta
Washington - Backstrom (19:31), Ovechkin (19:10), Oshie (17:52)
Columbus - Atkinson (11:00), Jenner (10:43), Wennberg (10:25)
NY Rangers - Stepan (24:46), Zuccarello (23:31), Kreider (19:43)
NY Islanders - Tavares (21:52), Bailey (20:15), Ladd (15:52)
New Jersey - Cammalleri (13:25), Zajac (10:28), Palmeiri (8:49)
Carolina - Aho (7:36), Stempniak (6:49), Stalberg (5:31)
Philadelphia - Schenn (8:37), Simmonds (8:03), Giroux (7:46)
Link
*Sort by "TOI with"