need help with this sentence 「あんたも鈍いね, あの子は脈ありだべさ 」

An example will help with 鈍い and 脈あり A similar line appears in Netflix’ 僕だけがいない街. Satoru’s mother is ribbing him for failing to recognize a possible love connection with a young woman who has just introduced herself. Satoru claims the young woman is merely a colleague from work. Mom calls him “鈍い” and declares the girl certainly has a “romantic interest” in him (quoting Naruto above).

The exchange goes like this: Satoru: ただのバイト仲間なんだから Mom: あんたも鈍いねえ。　あれは脈ありだべ。

Netflix’ English captions explain: Satoru: She’s just a kid I work with. Mom: You’re so slow. She’s into you.

So I interpreted 鈍い here to mean のろいん = thickheaded, obtuse, stupid; slow, dim witted, etc.

鈍い is the same as 鈍感 or "insensible", "dull". 脈あり and 脈なし are idiomatic no-adjectives that mean "having a chance / a sign of success" and "having no chance", respectively. These are most commonly used in romantic contexts, but can be used in other types of situations, too. Here are relevant definitions found in jisho.org. (There was no entry for 脈あり for some reason.)

鈍い

5. insensitive; dull (e.g. reflexes); unperceptive; unfeeling​

脈なし

1. just friends; no romantic interest; no pulse​