There are some restrictions in the airline I work for. For the gents this means:
Hair... Sides may be worn full, ears should be visible. Hair in the neck may be full and may slightly touch the shirt collar, however not the collar of the jacket. Extraordinary hair collors (blue, pink, green, etc.) are not allowed.
Sideburn... must be shaven correctly (tapered or blocked).
Beards... must be short and well kept, and may in no circumstance interfere with the use of the oxygen mask
Hairstyle for female crew members...
Same restrictions on the color as for males, long hair may either be pinned up or fastened at the neck by a clasp.
Visible piercings and tattoos are a no-no. If your tattoo can be covered and is not visible to others, than it is ok.
Generally speaking, I have never had any problems with any one in my crew about their grooming. Even with the rules written by the company, there is still enough room to express your personality up to a certain extend.
Nevertheless, crews wear uniforms for a reason, and the rest of their looks should be in accordance with the professional image the passenger is looking for in an airline crew.
Just my two cents.