it's either not gonna look like mlp or it's gonna look stuff ingame
When I see the pyro's necktie, I don't think "oh, that's from Killing Floor". When I see the frying pan, I don't think "that's the weapon from Left 4 Dead 2". When I see the scout's sword, I don't think "that's a weapon from that game nobody played called 'The Wanker' or something". All the promo items are generic enough that someone who's involved can catch the reference, and people who aren't can still appreciate the cool new item, and this would apply to any kind of hypothetical MLP item they put in.
ultimately there's a very, very low chance valve will accept a pony item, so I don't even want to bother arguing this
Obviously you did, or you wouldn't have started arguing about it.
ah that's true, they've trolled the stuff out of everyone so hard
If you think a few additional promo items are going to cause "controversy", then I'm going to encourage you to look at the reactions to the mann-conomy update, the free to play announcement, and the "halocaust".