Kinda depends how in-depth you want to get with the character and what says "feminine" to you.
By in-depth, I mean: are you going to take into account the actual demands of a mechanic's job and base the costume around that? This pretty much eliminates loose things, dangly things, frills, fringe, and beads, and possibly even dreads (unless they're short or well tied-back). There's a reason mechanics tend to wear coveralls.
You don't have to wear ALL of a coverall all the time, of course; undo the top and let it drape around your waist to show off your fabulous, immaculate blouse and waistcoat? Adapt a coverall to deal with (or be part of) a bustle gown?
Also, femininity. Does that mean frills, color, big skirts, etc. etc? Futzing with color is probably your best option if you're going for a 'real' mechanic.
And if you're not going for realism, all bets are off and the frills and fobs are back on the table. You can still suggest hard-working by choosing rougher fabrics and weathering the garments. A poofy skirt made out of layers of tatters? AWESOME. Well-made wool jacket, faded and starting to split at the elbows? ALSO AWESOME.
(weathering is probably the most important thing you can do across the board to suggest mechanic-ness. Use that soot and grease!)