Most people rag on her because she's beaten people way stronger than her. Let's call it ''joke induced stupidity'' for lack of a better term. She's still 100% legit though. Captain America's whole career is beating odds he shouldn't have been able to beat, and he's taken seriously. I don't know why Squirrel Girl wouldn't be taken seriously either. At her core, she's a comedic character with seemingly non-threatening powers. She also seems to have the same magic Steve had in regards to winning when she shouldn't. Good for her.
Besides, she's cute.