In general, no, because as a mechanical engineer you could be doing a universe of work that has nothing to do with pipefitting.
On the other hand, if you go into the mechanical engineering specialities to which you have already been exposed in the workplace, e.g. HVAC, process systems, etc., the specific experience might be perceived as a slight advantage, if for no other reason than you can communicate with pipefitters in their own vocabulary, using the correct rude words to describe problems and solutions.
The practical experience also implies that you are willing to get your hands dirty, and that you know which end of the wrench to pick up. This may or may not be a a good thing for your career. It may cause employers to assume that you can't do the blue-sky math that you will learn in engineering school, and to only consider you for certain kinds of engineering work. I.e., you may be typecast as "Marmaduke Surfaceblow".
Mike Halloran
Pembroke Pines, FL, USA