Likely a challenge, and Sakura will probably regret it ...or she'll probably continue laughing, only quieter. Tsunade's field of strength is medicine and with Sakura honing all available skills right now with Kakashi it will come much easier than it ever did it canon. I always thought Kakashi was a deadlier teacher than Tsunade, but Tsunade can keep an entire village alive on her chakra, so Sakura has a good deal to learn from Tsunade.

Could even get that seal on her forehead to store excess chakra sooner than expected, and much less of an asspull than in canon.