Deception of any kind, sometimes I wonder if monikers are considered an offense, using the Internet can be deception of any kind and a lie resulting in venial sin or even mortal if the deception is grave enough i.e. making yourself or your website/blog look better than it is by certain programming or buying "likes" according to what I have read.
I'm here to tell you: don't worry about "monikers" or screen names. That isn't deception. It's called choosing a screen name.
For that matter, going by a nickname IRL isn't sinful either. You can go by "slick", "bubba", "boogie", "slim", "TJ", "DJ", or whatever nickname you want.
If you were to tell someone, "My birth name is TradCatholic075." that would be a lie. Probably a venial sin, since that isn't a grave matter.
But just choosing a screen name is not deception at all. Unless you claim it's more than a screen name. If I choose the screen name "Matthew" and sign all my posts "Matthew" but my real name is actually "Tom", then yes I would be deceiving everyone. People would think it's my real name, but it wasn't.
It's also not sinful to go by your middle name or confirmation name. Many people do this, for various reasons.