Let me get straight to the point.
Comparing Kimi no Nawa and Tenki no Ko is like pitting a glass of wine and a mug of beer against each other. Both get you wasted, but in terms of subtle details and quirky characteristics, like taste and smell, they can be obviously distinguished from each other.
Tenki no Ko and Kimi no Nawa happen in the same universe, but, the usage of cartoons, symphonies and even theme are very different in both films. For example, Kimi no Nawa blended the music and visuals to describe two different backgrounds; the countryside and mainland Tokyo. It dwells on the theory of fate, where one meets his other half through supernatural means and grit. This is the film's main theme, where the environment is used to appeal than solidify as a breathtaking experience for most audiences.
On the other hand, though instilled with Shinto mythology, Makoto's imagination, and impressive symphonies , there is a hint of reality. Harsh weather of global warming sets in, where Makoto seems to be addressing his audience about this issue, while knitting it together with romance and pure spectacle. It may seem slightly more 'shallow' in terms of presentation and storyline, due to the fact that we only get to see our protagonist hustling through difficulties while living in a merciless Tokyo society, instead of countryside and the 'high-school life' that most of us got out of kimi no nawa.
For those still in this debate, here's a quote for thought:
" Empty your cup so that it can be filled; become devoid to gain totality." As a bystander, I do feel the reason why there is so much hate on Tenki no Ko is because many feel that it did not surpass Kimi no Nawa's 'level'. But do keep in mind that both films are separate projects, and comparing them may just lead to more distress.
So just empty your cup of expectations, and accept both films for what they are.