Some(A) bumper stickers on cars are(B) quite funny and make us laugh, but(C) another(D)are quite ridiculous.
There is nothing wrong in the given sentence with the phrase “Some bumper stickers". The plural verb are agrees with the plural subject stickers and its present tense is also consistent with the tense of the tense of the other parts of the sentence.
The conjunction but brings out the intended contrast within the sentence, and is quite appropriate. The word another which is in singular number is not appropriate for the contrasting with the plural word some in the beginning of the sentence. Additionally, being in singular number, it does not agree in number with the plural predicate are in the second part of the sentence.
So, the underlined word another must be replaced by the word others to render the sentence correct.