Smash is an animated game. Anything depicted has an essence of being a cartoon.
Looney Tunes didn't claim they were avoiding realistic firearms. There's really not too much more realistic they could have gotten.
Snake didn't get to use his gun and guns were also a reason for not including James Bond back in Melee (despite Goldeneye's popularity and acclaim).
One could say that Bayonetta contradicts this, but here's the thing: unlike Snake and Bond, her guns aren't realistic, sporting flashy designs and actually don't fire realistic bullets, but magic energy (Bayonetta is a witch, after all). Plus, wielding guns on her feet in addition to using guns on both hands is just too ridiculous to be taken seriously and far away from realism.
Speaking of Bayonetta, I realize more and more that, despite she's not an iconic third-party character (unlike Sonic or Ryu), she probably has the most ties to Nintendo out of the third-party games and that may have been a reason why Sakurai picked her and saw her as viable.
Think about it: Nintendo published and financed Bayonetta 2 exclusively on Wii U and even assisted on its development, with one of the producers being a Nintendo employee. The same applies to the Wii U port of the first Bayonetta game.
Along with the fact that Sega and Platinum Games have a good business relationship with Nintendo and already had some involvement in Smash (Sonic, Virtua Fighter costumes, The Wonderful 101 content), it made her a more negotiable character out of potential third-party characters.
She is still something of an exception to third-party characters in regard to the lack of legacy, but then again, she is the ballot character, so her inclusion is meant to be fanservice. However, I think fact that she was a character relatively easier to get into Smash from a business side of things helped with her inclusion.