Gumshoe relies so heavily on the Investigative/General skill split that I'm going to address them separately.
The core of Gumshoe. The points allocation suggested in Mutant City is moderately generous, but this is by design. Gumshoe is expressly about making sure the plot can't stall due to of lack of a relevant skill.
The group should collaborate in character design to ensure all or almost-all skills are taken, but it's not crucial to worry about how good you need to be at each - go with whatever fits the characters. It's inherent to the Gumshoe system that as long as all the investigative skills are covered you'll always get the information you need, and the actual skill point totals provide helpful bonuses, not required information.
So, short form: As long as you cover the range of skills, you can't have mis-designed the characters. If you do stall completely, it's the GM that's made a mistake, not the players.
Asking whether the default superpower allocation is generous or stingy is a somewhat relative question. What counts as 'a lot' of power in a supers game? Depends whether you prefer your superheroes at the 'Superman' level or the 'Daredevil' level. However, MCB's default power allocation fits pretty well with the theme and setting - powers make you significantly more capable than unpowered humans, but not so much that there's no interaction. You can still die of being shot. Unlike Marvel/DC, a typical character will have two or three related powers in a group at low-to-medium level rather than one awesome power. (The Quade diagram works well as a character creation tool, although it really shines in play as a forensic tool.)
Since the game is ultimately about policework, the level of power isn't too important - superpowers are bonuses to aid your police skills. (Unless the GM is going to focus heavily on fights with powers, in which case the PCs had better have some characters with 'classic hero' powers like Armor, Fire Projection etc.) But this isn't Marvel Heroes; you're the police. If massively outpowered, 'contain and call for backup' should always be the option.
(Note in particular that the Quade diagram makes the classic 'super-strong tank' in the Juggernaut/Rhino/Colossus vein impossible as a character. This is a deliberate and important design choice; MCB is not about having to slug it out with a villain that can smash everything and won't go down. You just can't get from Armor to strength or energy projection powers. Strength/Regeneration is possible... but won't win a fight against a normal human with a shotgun. And if built in Mutant City, Phoenix would be defeatably weaker... and autistic.)
Investigative powers make policework easier, general powers make fights and crises easier, and both can safely be picked based on what seems to fit the character. Although the group as a whole should try to have some of both, as you'd expect.
(Mutant City Blues would actually work really well to do an early 'Powers' style game, in which the PCs don't have any powers and chase single powered-villain antagonists. Just skip the power part of character creation.)
Errors to avoid
As far as I can tell, there aren't any - no beginner player in our group expressed any regrets about character choices. Honestly, it's quite hard to go wrong in Gumshoe.
Mutant City Blues has a forgiving game design. Make sure the investigative skills are covered; maybe put a few more points into the most obvious frequently-used skills (Law, Interrogation, Negotiation, Evidence Collection, Forensic Anthropology, etc.) If your game is going to contain arrest firefight scenes, take the usual care to have some combat-capable PCs - Health, Scuffling, Shooting, Sense Trouble. Other than that, you really can't go wrong.
If you're still worried - our group has a house rule for all new systems that you can reallocate your skills at the end of the first game. Try it.
There are some sample characters here that might help serve as a guideline:
Example PC Sheets
Don't miss the 'legal summary' reference sheets available from Pelgrane's site.