Her actions can also show her 'dirty mind'. Maybe she's always looking for a chance to play pranks that involve some kind of mild sexual innuendo. But at the same time, these actions should be in line with the fluff and light - hearted nature of the app, not too vulgar. For example, she might swap the labels on items in a way that creates a humorous and slightly risqué situation.
Another way is through her inner thoughts. The writer can give the readers a peek into her mind where she has these naughty ideas but then she might quickly censor herself or be embarrassed when she realizes how inappropriate her thoughts are in a particular situation. This shows her as a complex character with a 'dirty mind' but also a sense of propriety.
She can be portrayed through her dialogue. Using double - entendres or making cheeky remarks in response to normal situations. For instance, if someone mentions a 'pole', she might make a joke that has a naughty undertone.