The optics of this telescope are not bad. The lenses are properly coated, and the optical power is enough to view some major details such as Ring’s of Saturn, moons of Jupiter, and Mars as a red dot. The Moon shows a lot of details as it does with most other telescopes.
If there is a problem to be found in the optical design of this telescope, it is the short optical tube. An ideal refractor would have at least an f/8 focal ratio, which would require a much longer, 720mm optical tube. Gskyer 600x90mm, as you might understand from the name, has a shorter, 600mm optical tube. This results in color defects around bright images, called chromatic aberrations.
Other than that, the optical design is fine. It is not the best you can get at this price range, but it is not below-average either. If you are going for pure optical power, you should get the highest aperture telescope you can find. Aperture is king at the budget price range.