No, I don't think so. Some alternatives: "he couldn't help noticing that there seemed to be a lot of strangely dressed people around." "he couldn't help noticing that there appeared to be a lot of strangely dressed people about." "he couldn't help noticing that there were a lot of strangely dressed people about." "he couldn't help noticing that there were a lot of strangely dressed people in the area"

Is there a special structure with "seem to be ... about" in this sentence? No, there isn't. The word 'about' has nothing to do with the 'seem' structure. The structure is simply 'seem + to + infinitive' followed by the rest of the sentence. The word 'about' belongs to the phrase 'to be about' ( or 'to be around') meaning that these people were in the vicinity : the speaker noticed that there were a lot of strangely dressed people walking about or standing about in the streets, or wherever the speaker happened to be.

In this sentence, about = around. so, "there seemed to be a lot of strange people around" is also good.

In this sentence, about = around. so, "there seemed to be a lot of strange people around" is also good.