The trouble with clouds is...they're never perfectly shaped like a computer drawing is. In your example, the shapes are too good. Try using the pencil tool. I draw these all the time, but hardly ever create a totally closed shape, especially on the strokes. Try using non-connecting lines and maybe even the roughen filter so your graphic does not look too good. And, also try using small stroke lines ( along the edges ) to make the cloud appear puffy. You could also use a radial gradient using pastel colors and/or transparency. Let your mind fly, the list is endless on what you could do.
… And – last but not least – take the time to look at the real sky and try to remember what you've seen there before you reside in front of a computer.
There are various online tutorial for creating cloud from Illustrator.
Based on the intent, you need for a cartoon or need more realistic approach the steps and efforts may vary.
What you have designed with the ellipse tool I guess is nice for a cartoon illustration but for having a real cloud like look
making the variation based on my requirement.