To be fair, that was a cost-cutting measure -- the New Vegas DLC was infamous for having pages upon pages of dialogue, and they likely needed to make her mute to fit her into their budget, both monetary and time-wise.
I actually don't find Veronica terribly Mary Sue-ish. She's way less powerful than Boone, her Perk is one of the less useful ones, and if you aren't a melee fighter she can get herself into some awful pickles (which is a problem if you're playing Hardcore). Further, with the exception of McNamara, almost every other Paladin seems to want to straight-up murder her.
If you want to talk about a truly godawful Felicia Day video game character, let's talk Dragon Age II. Urrk.