To answer your first question:
There's a lot of layers to it. I'll try to keep myself relatively concise:

Before fluency, someone speaking their L2 is going to sound like a little child--they can only express themselves in short declarative sentences, stumbling over the wrong words, and unable to form certain phonemes.

For many monolinguals, they don't even think about how difficult it is to think in a new language. They've only had the perspective of one language all their life, and it just doesn't occur to them that thinking in another language is a tough task. It just seems like this person must be slow or stupid if they can't form a complete sentence.

Some people are just mean-spirited assholes who view their culture as objectively superior, and accents are a great big sign saying "this person can't speak my language properly, they must be stupid."

Cultures develop stereotypes of a certain type of people, and accents are a common identifier. A lot of people from the Southern US find it hard to be taken seriously when they speak 'naturally' because the Southern drawl has become synonymous with "country bumpkin." So, if your accent has a certain comedic connotation, you'll get painted with that whether it represents you or not.

There's probably other, less cynical reasons too:

I get the feeling that every language has innocent words that are next-door neighbors to some bad ones, and some accents happen to hit that spot halfway between the two. Maybe your accent changes a vowel from sounding like "shout" to "shite," and a listener can't hear "He was shiting up and down the street" and not giggle.

As for your second question as a language model I don't posses feelings or emotions and personal preferences. Saying that, laughing at accents within a human context may be consider sometimes inappropriate depending on the situation
spam fails_task lang_mismatch pii not_appropriate hate_speech sexual_content quality toxicity humor helpfulness creativity violence