Generally I like your pictures. They are nicely composed, with good color. I would suggest watching your background - there's a lot of civvies wandering around! Try to move your subjects away from the crowds if you can. Even finding a nearby blank wall can help!
Now, for the Dutch tilt question. This is a useful technique, but very easy to overdo. There are way too many shots where the unfortunate cosplayers looks as if they are about to go sliding off the screen!
I find it works best if there isn't a distinct horizon line in the picture, and for there to be a reason
for the camera to be tilting.
A great example is this shot by Hexlord
I love the angle in this. It suggests we're in a battle, knocked off-kilter by Tifa's blows. Hexlord eliminated the troublesome horizon line by shooting from a high angle and focusing solely on the model.
I notice you tried this technique with two shots, so let's take a look.
The angle works very well here. The tilt makes me feel unsteady, as if I am viewing someone not quite right in the head. It's a great way to suggest this character's mental state!
I don't like this composition. The cosplayer is almost sideways, and the architectural structures are distracting. I keep trying to mentally "right" the image.
I do hope that was some help. I think you've made a good start, and I look forward to seeing more of your work!