When I was working (technical) I'd wear a torn over-stretched, t-shirt old jeans, etc. including to customer sites (apparently I got quite a reputation in the industry), customers got to mostly accept it. One day I was dragged on a sales visit and had to wear a suit and had to call in at existing customer to do something on way home and I got a lot of comments (for being "smart"). They thought it funny seeing me in "fancy dress".
I always found with customers turning up comfortable as you are gives them the impression you are up-front and can be trusted. In sales meetings when sales person said something wrong I'd happily interrupt and say "no, that doesn't work ..." in front of customer ... so customer quickly got to believe anything I said, they'd end-up asking me questions rather than sales person and sales people were very happy as we'd win the deal as customers trusted they were getting truth.
So for me the important thing is to be who you are rather than pretend to be somebody you are not.
Ian