Learn Portuguese (Portugal): Esta cidade é mais aborrecida do que Londres.

To compare two things, we use mais (more) followed by the adjective, and then do que (than). Here, we have mais aborrecida do que (more boring than). You might sometimes hear people drop the do and just say que, but do que is the standard and most natural way to say it in Portugal.

Why does the adjective aborrecida end in an "a"?

Adjectives must match the gender of the noun they describe. Because cidade (city) is a feminine word, the adjective takes its feminine form, aborrecida, instead of the masculine aborrecido.

I learned that aborrecida means "bored". Why does it mean "boring" here?

The meaning changes depending on which "to be" verb you use! When used with the verb ser (used for permanent traits, like é in this sentence), it means "boring". If you use it with the verb estar (used for temporary states, like está), it means "bored".

Why do we use Esta instead of Isto for "This"?

Esta is specifically used before feminine singular nouns, like cidade. You would use Isto when pointing out an unspecified thing, an unknown object, or a general idea, but never directly before a specific noun like we have here.