When you use mimikyu against a trainer, the AI does not act smart to the disguise.
For example pokes will get swapped out eventho they could 2hko it easily. Usually the ai brings out a poke with a move like stealth rock/leech seed/toxic and it uses that after it comes in. Than in the leech seed case it keeps spamming that same move instead of something like sludge bomb (on venusaur) giving mimikyu a easy setup opportunity.

Thanks for the report. These things can be quite subjective at times as to the best move the AI should make, but I do agree with you that it seems a little off against Mimikyu, so I've passed it along to see if anything can be done.