Is JIm trying to hide his identity? (look like something or someone else)

I think the "a" changes the meaning. "Mike is Jim in disguise." would simply mean Jim is pretending to be Mike. "Mike is *a* Jim in disguise." implies this person thinks of the name Jim as a specific personality or something and thinks Mike's personality/characteristics make him seem like he should be named Jim instead.

Maybe Jim is supposed to be "gem", like "Mike is a gem in disguise". Meaning his worth is not apparent to everyone