Sure it is possible to grow great trees from seedlings -- many cultivars in the trade today are clonally reproduced from seedlings generated by other cultivars.

But there is never any assurance about the appearance or vigor of the offspring. They could be strong, vibrant trees with great form or coloring or they could be something less. But in general, if the seedlings are struggling or not performing well, I'd suspect something not quite right with the cultural conditions rather than with the 'quality' of the seedlings themselves.

One thing to keep in mind if you'd like to grow these seeds, is that they may not exhibit the parent tree's variations, more specifically the 'mutations' that caught the eye of whomever first saw it. You may get a very 'average' type of Japanese maple from the seed, or you could get one with some very interesting variations.

Norway maples, for example, have many, many varieties that are grown for their form or red/purple foliage. 99% of the time, if the seed is viable (not sterile) they will end up being green the way the original Norway maples were and not inherit that parent tree's beautiful foliage.

Japanese maples, no matter what you get, are so interesting and beautiful, but the only way to guarantee that you have a type that is identical to the parent tree is to take a cutting and have a clone.

I myself am the proud grower of a 'generic' red Japanese maple. It arose as a seed sprouted from a "Bloodgood" from a friend. I, personally, prefer what I have, as it has the bright red spring leaves and crimson fall color, but the summer leaves are a stunning vibrant bronze-green, with the tips still a bright red where there are newer leaves. I love it!