hmm, not sure if i should hand out easy ways to avoid little mac stuff, but there are a number of things you can do:
1. every time mac dtilts/utilts/jab/down throw, otherwise pop you into the air, jump. this prevents further follow ups on you. mac can only up b if he reads it (which is only dangerous if you are at kill percent), and double jump into an aerial. are you gonna care when you get hit with an aerial that does 5% that he might wiff which you can then punish?
2. abuse your shield, as little mac only has his dtilt/utlit/jab that is safe on shield, and it must be spaced for him. and if he grabs you...ok but he's only gonna do like 10%.
3. just play defensive. if you play a character like falcon, just wait until he throws out a smash attack, ftilt, or other unsafe move, then dash attack or grab and follow up on mac. the only moves that mac can throw out in neutral safely at all times are dtilt and jab.
while little mac has one of the best neutrals in the game, mac can't afford to make mistakes, or he will get punished. not to mention, if you take a percent lead, you don't have to approach mac at all, and can just wait for him to approach you. even camping is a viable strategy here (yes, its not likable, but if lets you win, ok.)
all this advice is coming from a little mac who has more or less mastered the character, knows macs frame data, follow-ups and strings, and has had numerous top 8's and some 4th places at my locals and 80+ monthlies. take it from me, you follow those steps, you will have an easier time.
That said, bowser junior struggles, because he has NO MOVES that are safe to throw against little mac, its kinda a bad matchup for him.