I think Guybrush's style is different in
MI1-MI2 and
Curse-Escape because of an art style change. I played
Curse (which still remains to be my favorite of all four games) before any of the others, so I didn't find his look a problem.
Also, something else to take into consideration is the missing chapter between
MI2 and
Curse. Who knows - he could've mixed up a voodoo potion at the carnival that turns him back into an adult, and some of the side effects were: loss of facial hair, blonde hair, and growing too tall.