Measure of harmonicity in a time-series
Using the standard deviation of time lags is not a bad idea - the problem is that for very noisy signals such as consonants you won't really get a pattern with peaks. Your suggestion would be more useful in the context of musical instruments sound (for example to measure the inharmonicity of a sound, from violin to piano to bell...)

You can look at the ratio between the value taken at the highest peak in the vocal range (say 80-400 Hz), and the value of the ACF at 0 - this is a common criterion used for voiced/unvoiced classification. Another simple metric that works relatively well with clean recordings is the zero-crossing rate.