Caster level is rarely a prerequisite for a magic item's creation
That is, a creator usually need not meet a magic item's listed caster level as a prerequisite for the magic item's creation. Instead, a magic item's caster level is the magic item's caster level for determining effects it creates (such as with a cape of the mountebank) and, for example, used to determine if the magic item's rendered inoperable for 1d4 rounds because of an effect like the spell dispel magic. A magic item's creator can, in fact, set an item's caster level to anywhere between the minimum caster level needed to cast the highest-level prerequisite spell for creating the item and the creator's own caster level. (Of course, this being D&D 3.5, exceptions exist—armor, shields, and weapons bearing magical enhancement bonuses, for example, do have caster levels as prerequisites as do a fair number of other items.)
So, while "[a]dding one of these [common item] effects to an existing item works much
like creating an item from scratch[, t]he crafting character must [among other things] meet the given prerequisites" (emphasis mine) (Magic Item Compendium 234). And those given prerequisites are the prerequisites listed under the heading Prerequisites on Table 6–11: Adding/Improving Common Item Effects, and, of those, only the resistance bonus on saving throws has a caster level prerequisite. The other effects simply don't.
But there are two reasons it's kind of okay there are largely no caster level prerequisites for even high-powered common item effects:
The core rules were kind of random on whether a magic item equivalent to a common magic item effect should have a caster level prerequisite in the first place. For example, a ring of protection +1 has a prerequisite caster level, but an amulet of health +6 doesn't; an amulet of natural armor +1 has a prerequisite caster level, but while the magic armor special abilities greater, improved, and regular ol' acid resistance probably should, they don't. With this in mind, the Magic Item Compendium's Table 6–11 is more regular than the core rules.
High prices prevent most low-level characters from making game-breaking magic items. For example, to add to whatever face or head slot magic item the creator already has the common item effect +6 enhancement bonus to Intelligence has a price of 36,000 gp (not the 20,000 gp the chart implies—the game assumes the creator's going from a +4 enhancement bonus to a +6 enhancement bonus, not adding to an existing item an enhancement bonus between +4 and +6). That means it's 18,000 gp to add that effect to another item, so it'll consume just about all of a level 7 PC's gp. Using DMG guidelines (199), it's not until level 12 that a PC's wealth by level allows even a constructed character (i.e. one that enters the campaign at a level above 1) to spend at least 18,000 gp on single magic item the PC himself creates.
This means, if a caster level 3 level 11 character took the feat Craft Wondrous Item in a campaign I were DMing, I would let the character add to his cape of the mountebank the common item effect +5 deflection bonus to AC for a retail price of 50,000 gp or a creation cost of 25,000 gp if he could meet the other (admittedly, really lax) prerequisites, despite the character not meeting, for example, the caster level 15 prerequisite of a ring of protection +5. (I have some bias here, though: that adding such an effect takes nearly two months is usually a much bigger deal in campaigns I DM than either the XP cost or the gp cost!)
By the way, this isn't a rules-as-written question
Remember that the whole point of the Adding Common Item Effects to
Existing Items rules are to make the game more fun:
One of the most frustrating roadblocks to using interesting, unusual magic items is that they take up body slots that you need for an ability-boosting item… or another must-have item. (233)
But if this was not one of the game's frustrations before, and, now, because of these rules, the game is less fun, then these rules just shouldn't be used. If, for example, the DM wants PCs to seek out high-level casters and have the PCs commission those casters to make high-powered magic item on the PCs' behalves instead of having the PCs make such items themselves, that's perfectly reasonable in the right campaign. Like most rules, the Adding Common Item Effects to Existing Items rules aren't for every table.