Hi Kazal, It could be one of a couple of things, but the most common cause of spindley stalks is when the rhubarb crown has been planted too deep into the soil - If you had thick stalks last year, it could be that the manure has covered too much of the crown so scrape a bit back off and wait and see.
Another reason could be a touch of crown rot from the winter, but if it was insulated with the manure over the winter, this should not be a problem.
I'd keep going with the manure, but don't cover the crown too deep with it, fleece it over winter (if you live way up north only), and divide the plant if the crown is getting 'crowded'. You could also try 'forcing' it for a while, as people have reported a better/ thicker crop later on after forcing the early seasons stalks.
Hope this helps, all the best.