Unfortunately, Verant did not give us an option to view target's surname in the UI. As far as why NPC's surname shows up in the target window, the answer is easy - NPCs don't have Surnames! Here's a little lesson many of us knew from either a) times when the NPC's real name had an EQType or b) our pas Guiding experience
The EQ engine is built not to show specific characters in PC/NPC/mob names. These characters are numbers and underscores. "A Blackburrow Gnoll" is actually named something like "A_Blackburrow_Gnoll08" and "Guard Ostyn" is actually "Guard_Ostyn00", where the underscores are shown as spaces and numers at the end get stripped out. (The numbers are used to distinguish between mobs that share identical names and can range from 00 all the way to 99).
What this means is that NPCs and mobs don't have a surname, they only have a first name. "Trumpy Irontoe" is actually "Trumpy_Irontoe00". PCs, however, have both first name and a surname, where first name is unique to each player (hence no need for digits at the end) and surname which doesn't really mean anything (in terms of character's attributes).