No need to tip. It is not like it used to be in the 50's when 3 or 4 happy smiling attendants would voluntarily come out and check your oil and main fluids, clean all glass, pump your gas and refuse a tip. We must pump our own gas in most places these days, and I prefer it that way. But only if voluntarily offering. Mechanic I would not, but it is up to you.
Some people have special relationships with their mechanic.
I take my car to the dealer all the time and those guys get paid very well, so I do not tip them. However if I took my car to a gas station mechanic and they do a good job I would tip handsomely.