Pichu should never corner anyone unless You plan on baiting them to come out of the corner. he has no real options besides shield, movement, or trying to bet them with f-smash(bad) or wl upair. maybe if You think they won't relize You can't do anything they'll jump over You and You can get a grab b-throw/ up-throw.
pichu only does a few things, not sweetspot, nair oos sheild, reveavse grab, sheild pressure(2 frame sheild adavatage with higher port), juggle, tech in place/get up-attack, juggle/not get juggled(better than puff(knew main)),nair/up wl AND emptys(ies)/cause whiffs.
if You're approaching with pichu only attack 1/4 the time and be more techincal than who YOu,re fighting cause everytime they press A You have to sdi/asdi/di(3 inputs per their one)
that said by not attacking You won't get 4 stocked as much just make 2-3 reads in advance even if they are respawning dd around them force two misses get a grab/up-smash.
if all this is done with perfect counter spacing You can do decent against hard match-ups sheik should always have perfect timing/spacing(important) for stuffing out everything You do but never land one because You pull back.