Chuncan Feng has stated correctly that past participles used as adjectives use "more," not "-er" for comparisons, regardless of the number of syllables that the adjective has. We would say, for example, "more bored" (NOT "boreder") and "more loved" (NOT "loveder") and "more worn" (NOT "worner") as in:
Trying not to fall asleep in the class, Henry was more bored than he had ever been.
Nobody could have been more loved than that baby. Her parents had waited twenty years for her to be born.
Shoes that are more worn are also more comfortable
It is true that one-syllable adjectives are usually compared with "-er," not with "mose," but the past participle usage of "more" + the adjective instead of "-er" added to the adjective is an exception.
It is interesting to note, too, that the "more" or "-er" choice for comparisons is not absolute. For example, a few adjectives can be compared using either, such as "quiet" (more quiet, quieter), "polite" (more polite, politer) and "pleasant" (more pleasant, pleasanter), as well as some adjectives ending in "-ly": "friendly" (more friendly, friendlier) and "lively" (more lively, livelier).
The longer adjectives, like "interesting," are, as you know, always compared using "more." Some two-syllable adjectives, too, take "more," and not "-er," such as "public," and "active."
Surprisingly, some one-syllable adjectives – in addition to the one-syllable past participles -- also take only "more": cross, fake, ill, like, loath, prime, real, right, worth, and wrong*. We would NOT say: "John is iller than Mary," or "Jack is wronger than Bob," for instance.
As for "tired," the Cambridge Grammar of the English Language* states: "A marginal exception [to the –ed rule] is tired , though more tired is much more usual than tireder."
English! Full of exceptions!
*The Cambridge Grammar of the English Language. Cambridge University Press. 2002