Hey, for a first attempt, that's not bad at all - well done.
The key to airbrushing mottles is practice - lots of it !
Mottles are one of those things which can be a real pain to get right, for anyone. Sometimes they work beautifully first time, and at other times can come out exactly the way you didn't want it to!
A lot depends on the thinning ratio of the paint, pressure, and distance from the surface being sprayed, and, of course, the fine-line capability of the 'brush being used.
The main reason I bought my first 'brush was for mottles, and I only used it for this, and a final clear coat.