Yes, Tails is a mechanic. To us, it seems ludicrous that a fox with two tails, who can fly like a helicopter and run really fast, would become interested in fixing vehicles that he simply never needs to use. Then again, the whole Sonic series lost a lot a credibility when Sega put Sonic into a car, when--according to his web bio--Sonic can actually out-run most high-end sports vehicles. Of course, in Sonic All Stars Racing, Tails was put into a plane. That he doesn't actually need to use. Maybe he's just really lazy.
We see Tails doing surprisingly little mechanic work too, although he does tinker with a few engines in the Sonic anime. Actually, we swear we saw him rummaging through our bins last night, scavenging for some left-over curry. Would we trust him to replace the oil in our car? No, no we would not.