One simple way to evaluate a quarterback is to look at how good the team's offense is. That's a good approach to take with Tebow, since he plays such a large role in the offense even when he's handing off, which won't show up on his stat line.
So how good has Denver's offense been in Tebow's 7 starts? They've scored 21.6 points per game, versus a league average this season of 21.9 ppg, so they're about average there (they'd rank 20th). In yards per game they're at 323, versus a league average of 345, so they're a bit below average (also 20th). Points and yards aren't perfect stats, but they're pretty good measures of an offense, they're simple, and they tell a similar story here.
We can also look at the numbers from
Advanced NFL Stats (move the slider to include only weeks 7-13), which do a better job of isolating the contribution of the offense (as explained
here). The Broncos offense has -28.2 Expected Points Added over Tebow's 7 games (4 points per game below average), which is actually near the bottom of the league (it ranks 29th for that 7-game stretch). But in Win Probability Added, Denver's offense is at +0.37, which is slightly above average (ranking 16th).
There's some inconsistency between the different stats (since Win Probability Added gives them a ton of credit for game-winning touchdown drives, while Expected Points Added doesn't treat those drives as any different from any other touchdown drive), but on the whole the picture seems to be a slightly below average offense. Since the rest of their personnel isn't great, Tebow has probably been pretty close to average at QB over his 7 starts, although he's doing it in an unconventional way.