This is a common phrase. It is casual. It is a friendly, slightly funny phrase. Example; I'm a sucker for a used book store. I always look for my favorite authors. The phrase would not be used while speaking to a doctor about your health. "I'm a sucker for fried fish and potatoes." If you are explaining something to your doctor it is too casual to use this phrase. Instead, say, " I really like fried fish and potatoes and eat them whenever I can."

As someone who is a native english speaker and uses it everyday all day, it is quite a common phrase. You might hear it used more in music rather than average conversation, but it can be used often!

You hear it a lot here in England. I’m a sucker for a chocolate biscuit on my teaching breaks!

The phrase "to be a sucker for something" is quite common in casual and informal spoken English, especially in the US. Native speakers often use it to express a strong, almost irresistible liking for something, usually in a lighthearted or self-deprecating way. For example: - I'm a sucker for romantic comedies—I just can't help myself! - He's a sucker for good coffee; he'll pay anything for a quality cup. It's not typically used in formal writing but works well in conversations, social media, and personal storytelling. Tip: Use this phrase when talking about personal preferences in a fun, informal way, but avoid it in professional or academic contexts. A British equivalent of "to be a sucker for something" is "to have a soft spot for something." For example: - I have a soft spot for classic cars—I just love them! - She has a soft spot for chocolate cake and can never say no to it. Tip: This phrase sounds natural in both casual and semi-formal contexts, making it more versatile than "a sucker for."

It’s common but it has an additional suggestion that the thing you like is a guilty pleasure that perhaps you should not like. I’m a sucker for freshly baked cookies. (Yes) I’m a sucker for a good night’s sleep. (No)