"Go early in the day and dress according to your plan".
All of Madame Tussauds' wax museums are worth a visit. FYI, they are similar but different. They all carry a local flavor, in this case Thai personalities I was not familiar with. This is a good thing.
Bangkok's museum opens at 10h00 and it gets busier as the day goes by. There are dozens if not hundreds of opportunities for pictures or selfies. Some characters are very popular while some are extremely popular.
The wait for a picture opportunity of you holding court with Her Majesty Queen Elizabeth or with President Obama or President Putin could be very long when it gets busy.
Same for professional sports figures. This brings me to my second recommendation; dress according to what you have in mind. If undecided, play it safe and dress smartly.
The museum is located within the Siam complex, with two or three massive upscale shopping plazas. The Sky Train (called BTS) is the best way to get there,