Learn Arabic (Modern Standard): باريس أفضل من لندن من حيث تصنيف المطاعم السياحية. - Paris is better than London in terms of the ranking of the touristic restaurants.

No. The word حيثُ is indeclinable, meaning its ending never changes. It always takes a damma on the final letter, even when it follows a preposition like من. Together, من حيث is a fixed phrase meaning 'in terms of' or 'from the perspective of'.

In formal grammar, isn't حيث supposed to be followed by a full sentence? Why is it followed by the noun phrase تصنيف المطاعم here?

You have a good memory! In strict Classical Arabic, حيث must be followed by a nominal or verbal sentence. However, in Modern Standard Arabic, it is very common and accepted to follow it directly with a noun or Idafa phrase to mean 'in terms of [noun]', as seen in this sentence.

Cities like Paris and London are treated as feminine. Why is the comparative adjective أفضل in the masculine singular form?

Whenever a comparative adjective is followed directly by من (than), it stays locked in the masculine singular form. It does not change to match the gender or number of the nouns being compared.

Why does the adjective السياحية have the definite article while تصنيف does not, even though they are part of the same concept?