Was any president reelected with a smaller margin of votes than the first time he was elected?

yes- Woodrow Wilson who won 435 electoral votes in 1912 but only 277 in 1916. (However, this is misleading since Wilson actually got more popular votes in 1916, but in 1912 the Republican vote was split by Roosevelt and Taft and in 1916 there was no split. It seems the voters either like the president and return to office with a bigger margin or they decide to vote him out of office.)