Why would you want to explain, and hence highlight, your shortcomings? The personal statement is the one place in which you can let your individuality and uniqueness shine through, and thus hopefully win over an adcom. Unless you have really messed up and fallen all over your transcripts then I see no need to squander that space in the application. There is a section, however, that you can list any special circumstances or obstacles that might have delayed your graduating or caused great difficulty in your academic life. Perhaps that would be a better place to expound on such things than in the statement. Best of luck!